It has been two whole years since Bruce Banner last transformed into the Hulk. In that time, he has served as the Avengers' resident tech guy as the other members are out on the field, and has grown much closer with his teammate and fellow outcast Wanda Maximoff.
One morning, mutant terrorist Juggernaut attacks a military base in upstate New York. Wanda and Pietro arrive on the scene in an Avengers Quinjet, with Bruce assisting them from back at the Mansion. Despite Bruce and Wanda's warnings, Pietro rushes into battle against the Juggernaut. He attempts to tire the brute out by running circles around him, but is thrown up into the air when the Juggernaut simply slams his fists into the ground. Pietro recovers, and decides to charge head-on into Juggernaut, speeding off as far away as he can, and then aiming right for the villain's gut. However, Pietro simply runs headfirst into the Juggernaut and knocks himself out. Wanda tries to hold back the Juggernaut with her energy projection. She nearly overpowers him, but he starts taunting her, saying that no matter how many humans she saves, they'll always see her as a freak. Wanda briefly freezes up, giving the Juggernaut enough time to burst through her force fields, which sends her flying back. The Juggernaut looms over a weakened Wanda, and Bruce realizes she's not going to be able to stop him alone. With Iron Man and Captain America both busy, Bruce does the only thing he can, and quickly arrives on scene in a Quinjet.
With the Maximoffs defeated, the Juggernaut continues to rampage through the base, fighting off soldiers with ease. Suddenly, he hears a plane coming in overhead. He looks up, and sees another Quinjet, flying straight toward him. The Juggernaut winds up a punch, but the Quinjet hits him head on, with enough force to knock him down to the ground. The Juggernaut looks up, and sees, rising from the wreckage, the Hulk, who looks angry as all Hell. The Hulk charges at Juggernaut, and manages to send him flying with a single punch. The two brutes fight against each other, with the Hulk quickly overpowering Juggernaut, prying off his own helmet and bashing it into his head. The Juggernaut is incapacitated, but the Hulk continues to pummel the mutant. A recovered Pietro tries convincing the Hulk to stop, but is instead sent flying with a massive kick. Wanda catches her brother in the air and levitates him down to the ground. Wanda tries talking sense to the rampaging Hulk. She briefly gets through to him, but the beast leaps off into the woods, fleeing the scene.
That night, the Hulk sits by a lake out in the wilderness. He starts to tire, and begins hallucinating. First, he hears the voice of Betty Ross in his head, from when she first confessed her love for Bruce Banner. The Hulk opens his eyes, staring down at the water, and sees Bruce Banner in his reflection. Bruce tells the Hulk he has to go back, that he can't stay Hulk forever. The typically quiet Hulk grumbles out a sentence: "Hulk just wants to be... Hulk." Bruce says he understands. The Hulk starts falling asleep again, but before he can revert back into Bruce Banner, he hears planes overhead. The Hulk awakens, and looks up into the sky to see several Quinjets flying over him. Iron Man lands down, and tells Hulk that he had his fun, but it's time to come home. Hulk tells "Metal Man" to leave him alone, and leaps to attack him. Iron Man quickly activates several sonic cannons on his suit, which bombard the beast with overwhelming soundwaves, which just barely manage to incapacitate him.
Bruce Banner awakens at Avenger Mansion, Tony Stark at the side of his bed. Tony jokes about having to take Bruce's "Hulk sobriety" chip away. The two talk about the incident, with Bruce telling Tony that for the first time, he thinks he was able to communicate with the Hulk, inside his mind. Tony tells Bruce that he was planning on telling him this over a nice dinner or something, but he might as well break the news now. A scientific research organization known as A.I.M. has created an experimental procedure that they believe can "cure" Bruce of his specific condition, meaning he will never be able to turn into the Hulk again. Bruce is unsure about it at first, so the next day he and Tony meet with George Tarleton, one of the head researchers at A.I.M. Tarleton explains that Phillip Sterns, the organization's founder and the brother of Samuel Sterns, Bruce's former nemesis, spearheaded the procedure, seeing as his brother was technically responsible for the creation of the Hulk, and he felt this could be his way of making up for everything Bruce had to go through. Bruce comes to like Tarleton, and decides to go through with the procedure.
As Bruce prepares to fly out to A.I.M. Island, his cousin Jennifer "She-Hulk" Walters arrives at Avengers Mansion, choosing to accompany him on the trip along with Wanda and SHIELD agent Clay Quartermain, who awkwardly tries to hit on Jennifer. Tony and Pietro bid Bruce a farewell, and he sets out for what he hopes will be the last day of his life as a monster. Arriving on the island, Bruce is greeted by the newly married Betty and Leonard Samson, who told Tarleton and Sterns they wanted to surprise him. Bruce is happy to know that they contributed research for the procedure, but it is a bittersweet reunion. Bruce meets with Sterns, and the two hit it off almost immediately, bonding over science. In contrast, Jennifer doesn't seem very interested in getting to know Wanda. Rather, she seems suspicious about A.I.M., and what their real intentions are. In the lab, as Quartermain is fixated on an unusually large reptile in containment, Tarleton and Sterns break down the procedure for Bruce, telling him that A.I.M. plans to use their experimental "Nega-Rays" to try and reverse the gamma mutation in his body. A large machine will absorb all of the gamma energy Bruce's body will generate as it tries transforming, and, once the laser show is over, only Bruce Banner will remain, and the Hulk will be no more.
That night, Bruce is trying to get some sleep before the big day. In an idyllic dream world, he is walking with Betty through Times Square, the two of them happily married, just as Bruce always wanted. Suddenly, a crowd of scared civilians rushes past them. The ground starts to rumble, and the Hulk bursts through the front of a building, yelling for Banner. Bruce holds Betty in his arms, promising her it'll be alright, but she vanishes. Everything around Bruce fades away, leaving him in a desolate desert. Everything except the Hulk, who charges at him, telling him that "Hulk's body is Hulk's." Just before the green brute can crush him, Bruce wakes up, gasping for air. He looks up at a painting on the wall of a beautiful landscape, and tries to focus on that, as to cool down from the stress of the dream. On the other side of the guest building, Jennifer wanders out of her room. She decides to take an unauthorized stroll around the island, and comes across a large building with two armed guards roaming the perimeter. She finds this suspicious, and decides to sneak into the building, going the other way around. Inside, she comes across a large conference room, and watches from the shadows as Tarleton and Sterns stand before a large monitor displaying wanted fugitive Obadiah Stane, who is in some sort of underwater research facility. Stane tells Tarleton and Sterns that they need to get the procedure over with quick, as their current power source is draining and without a steady supply of gamma energy, the "plan" is never going to come to fruition. Sterns promises him that they'll have the procedure done and their next power source secured by tomorrow morning, Tarleton adding that their cover-up story is fully prepared. Two A.I.M. guards enter, and spot Jennifer. They open fire, and she manages to evade the bullets. She knocks out one of the guards, but more rush in, and Sterns tells her that if she doesn't surrender, it's going to get ugly.
The next morning, Bruce eats his breakfast. Wanda approaches him, and asks him if he's ready to go through with this. He's as ready as he'll ever be. Agent Quartermain meets up with them, and asks where Jennifer is. Bruce has no idea, but he doesn't want anything to delay this. They arrive at the laboratory, where Tarleton and some scientists hook up Bruce to a large device, Tarleton adding that even at his angriest, the Hulk wouldn't be able to break through it. Both the Samsons wish Bruce good luck, and step aside as the procedure begins. Wanda looks on, clearly worried. With a grin on his face, Sterns activates the sequence. In a secret holding cell, Jennifer awakens. She looks out into the corridor, where two guards get a call on their radios, stating that the procedure has begun. Jennifer grabs onto her cell door, channeling enough strength and anger to throw it off the hinges. The guards attack her, but she takes them down with ease. As more guards rush into the hall, Jennifer fights her way out of the building, kicking through a wall and making a run for the lab. As the Nega-Rays fire on Bruce, his skin starts turning green and his body mass begins to rapidly grow. Wanda takes a deep breath, while Quartermain gets a kick out of the spectacle of it all. Bruce transforms fully into Hulk, but as Tarleton predicted, he is not strong enough to break out. Suddenly, Jennifer burst into the lab, taking out all the guards, and throwing one into the control panel with all her strength. This causes the sequence to go haywire, and the Hulk is overloaded with radiation. Everyone takes cover behind Wanda as she throws up a force field, protecting against the giant explosion.
As the dust settles, they all look up to see the entire lab completely obliterated. The Hulk is seemingly destroyed, but in reality, buried beneath a pile of rubble and ash, he is back in the dreamscape. Both Hulk and Banner are dazed and all out of sorts. They rise to their feet in the desert, as a large shadow looms over the both of them. Back in the lab, all of the on-lookers are completely speechless, until the wreckage begins to shake. Exploding out of it comes a larger, stronger, angrier, paler Hulk. The Grey Hulk. A.I.M. guards rush in and open fire, but the bullets just bounce off of him and ricochet all over the lab, one taking out Sterns' eye. The Grey Hulk grabs Betty Samson, and flees from the building, heading into the island's large jungle region. Guards try to apprehend Jennifer, but Wanda defends her, and the two run off. Leonard Samson goes with them, and tells them he has a device that can track the Hulk's gamma signature. Samson turns on the device, and sees that, because of his more powerful new form, the Hulk is easier to track than ever before. He gets into a vehicle with Jennifer and Wanda, and they drive off. Sterns tells Tarleton to activate Stane's robots. Along with several A.I.M. guards, a fleet of fully mechanized androids is sent out after the fugitives. Quartermain is caught trying to escape by some guards, and is taken to Tarleton and Sterns.
The Grey Hulk carries Betty up into the mountains. She tries to get through to him, asking him why he's doing this. The Grey Hulk, a better speaker than Green Hulk, tells Betty that he is saving her. She talks him into setting her down, and when he looks away, runs off. Eventually, she is spotted by some sort of large animal. She looks, and is pounced upon by a large, mutated mountain lion. The Grey Hulk arrives and fights it off with ease. Grey Hulk promises not to hurt Betty, and she reluctantly goes with him for protection. They take refuge in a cave, where Betty is able to converse more with the beast. She comes to the conclusion that Grey Hulk must be a fusion of both Banner and the Green Hulk's ids, as the creature seems to be driven only by it's other persona's base desires. Driving through the jungle, Wanda and Jennifer come across a large group of mutated mountain lions, which seems to be what Leonard's device was leading them towards. Leonard points out that these animals seem to have been exposed to gamma radiation, but he has no idea how. The lions spot their vehicle, and attack it. Jennifer and Wanda join forces to fight some of them off. The rest of the lions flee, and Jennifer believes they have scared them off, until the A.I.M. robots land down behind them, attacking. Wanda manages to hold them off, allowing Jennifer and Leonard to flee. With the robots subdued, Wanda manages to generate enough energy to fly off after her allies. The A.I.M. soldiers shortly arrive at the scene of the battle, and contact their superiors, telling them that they nearly captured the fugitives, and that they just need more time. Sterns lashes out at the headquarters. Observing his tantrum, Quartermain asks Tarleton if he does this all the time.
In the mountain cave, Betty and Grey Hulk watch the Sun go down. Betty notices the Grey Hulk is exhausted, and tries calming him down. The Grey Hulk falls asleep, but he does not revert back into Bruce. Betty begins to fear that Bruce may be stuck like this, forever. In the Grey Hulk's mind, the creature roams through the desolate desert, eventually coming across a single flower. Grey Hulk pulls it out of the ground, and tries to get a closer look at it, but accidentally crushes it with his giant fingers. The Grey Hulk looks and sees Bruce Banner standing before him. Banner tries talking down to the creature, but the Grey Hulk tells him to shut up. Banner persists, trying to talk some sense into Grey Hulk, but only angers him. Grey Hulk attempts to attack Banner, but Green Hulk jumps out of nowhere, tackling him to the ground. The two Hulks fight, with Green Hulk nearly getting the upper hand, until Grey Hulk outsmarts him, punching him in the gut and sending him flying onto his back. The Grey Hulk stands above the defeated Green Hulk, and begins to repeatedly smash him into the ground. A sleeping Betty is awoken by the Grey Hulk thrashing about in his sleep. She is nearly killed by the raging beast, and runs out of the cave. The Grey Hulk wakes up, and runs after Betty.
Jennifer, Wanda, and Samson come across what Samson is sure to be the real Grey Hulk. Instead, they arrive at a large, green, irradiated lake. She-Hulk stands at the edge of it, and spots something large at the bottom. Since she is already a gamma mutate, she decides to dive in so she can see what it is. Swimming down towards the bottom of the lake, Jennifer is attacked by a large mutant shark. Seeing she is in trouble, Wanda creates a protective force field around herself and goes down into the water, saving Jennifer. As the two of them are down in the lake, Samson is surrounded and captured by A.I.M. soldiers. The two superheroines reach the bottom of the lake, and discover a large submerged facility. Entering the underwater base, they discover a large, heavily-guarded laboratory, where Obadiah Stane and some scientists are working on computers near a gigantic machine, which gives off a bright green glow. Jennifer spots a shadowy figure at the center of the glow, but just then, she and Wanda are apprehended by henchmen.
Out in the jungle, Betty hides from the Grey Hulk, who desperately searches for her. Suddenly, A.I.M. soldiers arrive, guided by Samson and his tracking device. The A.I.M. soldiers attack Grey Hulk, while Samson runs off, finding his wife. As Grey Hulk battles the soldiers, they manage to lead him to the edge of a cliff. One of Stane's robots flies in from the treetops, and attacks Grey Hulk. The robot briefly overpowers Grey Hulk, pushing him back over the cliff, but not before Grey Hulk can tear the machine in half. Grey Hulk plummets to the bottom of a deep chasm, and is finally knocked unconscious. The A.I.M. soldiers peer over the cliff, but can't see the bottom. They try to get the gamma tracker, but Samson is nowhere to be found. They contact Sterns and Tarleton. Enraged, Sterns decides to head out in an A.I.M. fighter jet and find the Hulk himself. In the underwater lab, Stane confronts the captured Jennifer and Wanda. They ask him what's going on, and Stane figures that, since he's going to kill them anyway, the least he can do is make things clear. He reveals that, inside of the gamma generator that powers A.I.M. Island, is Samuel Sterns, Phillip's comatose brother, who he obtained from SHIELD custody by claiming he would cure him. However, Samuel can only generate so much gamma power for them. The Hulk, on the other hand, can generate seemingly limitless energy, as long as they keep him angry. Their cover story was that they would accidentally kill Banner while trying to "cure" him, but in reality they sought to turn him into a living battery, not just to power their island, but to power a "super-weapon" of sorts. Jennifer sighs, stating that there's always a super-weapon. She jokingly asks him if he plans to blow up the world. Stane reveals that the plan is not to destroy the world, but to shut down all of the world's defense systems so that A.I.M. can conquer every nation, establishing a new world order in which the intelligent rule over the simple-minded. When they mock his plan, Stane says that he didn't come up with it. Sterns and Tarleton's "Intelligencia" did. He's just working for them because he has nowhere else to go, but he can't let them down. Before Stane can kill them, Wanda manages to muster up enough power to explode out of her holding cell. She tears through the robots and guards with ease, knocks out Stane, and frees Jennifer. Jennifer asks why she couldn't have just done that sooner.
In his jet, Sterns spots the Samsons fleeing through the jungle. He activates the jet's guns and opens fire on them. The two of them take cover, as Sterns readies the jet's heat-seeking missile. Before he can fire, something jumps out and grabs the jet, smashing it into the side of a nearby mountain. Sterns, severely injured, crawls out of the ruined jet, and looks up to see the Green Hulk standing above him. The Samsons are actually happy to see the original Hulk, and run to him. The Green Hulk smiles upon seeing Betty, and then slowly reverts back into Bruce Banner. Down in the underwater facility, Jennifer and Wanda work to shut off the power generator. They do so, and a large blackout spreads across the island. However, on top of the machine, the Leader awakens. Jennifer tells him she'll free him if he turns himself back into SHIELD peacefully. Instead, the Leader uses his mind control powers and possesses Wanda. Jennifer realizes what she's gotten herself into, as the Leader-controlled Wanda starts to tear the whole place apart. Jennifer tries talking sense into her, but water starts flooding into the island's power generator.
Back on the surface, Bruce and the Samsons wander, trying to find a vehicle, when the ground begins to shake, cracking open as green light shines through. The three of them are forced to run back to the wrecked jet, where they find the injured Sterns. He tells them that something's wrong with the island's gamma generator, and now the whole thing is going to blow. Sterns leads them to a secret tunnel that leads to the underground lab, only so they can all avoid a fiery death. As they journey down to Stane's lab, rubble falls down and blocks their way. Bruce realizes he has to turn back into Hulk to move it. However, the Green Hulk isn't strong enough to make it budge. A look of realization comes across the Green Hulk's face, and then he proceeds to push harder and harder, mustering up more anger and strength, until he transforms back into Grey Hulk. Finally, he is able to burst through the rubble with ease. The four of them enter the lab, where they see Jennifer dodging the mind-controlled Wanda's attacks. Sterns realizes his brother has woken up. As the whole place starts to fall apart, Sterns sees a recovered Stane getting away in an escape pod. Sterns runs through the lab, and gets into his own escape pod. However, before he can blast off, the possessed Wanda spots him, and levitates the escape pod into the air, crushing it with him inside. She throws the crushed escape pod at Grey Hulk, who manages to catch it. Seeing Wanda floating up in the air, the Grey Hulk freezes up. In his mind, Grey Hulk confers with Banner and Green Hulk, with Banner stepping forward. The Grey Hulk reverts back into human form, and Bruce tries to get through to Wanda. Jennifer runs over to the Samsons, and tells them they need to evacuate the island.
Back on the surface, Tarleton stares out a window, and realizes everything's about to blow. Two unconscious A.I.M. guards fly into the room, and Jennifer enters. Tarleton grabs a gun, and opens fire on her, telling her he's not going down without a fight. Jennifer grabs one of the doors, and blocks the bullets. She punches Tarleton, and he goes flying out the window. More A.I.M. agents rush in, but Quartermain lifts up the door and knocks one of them out. The other ones shoot at him, but Jennifer resuces him just in time, and thanks him for his efforts. She defeats the guards and rushes Quartermain out of the building, as the Samsons lead out the rest of the innocent people on the island. Back underwater, Wanda manages to overcome the Leader's influence. Bruce and Wanda embrace, and head for the escape pods. As everyone else rushes in, Bruce tells Wanda it's more important to get them into the pods first. As everyone starts blasting off, the ceiling starts to cave in. Wanda tries holding it off with her force field, but Bruce tells her she has to go. She calls Bruce crazy, but he tells her that, for once, he finally feels sane. She drops her force field and is pulled into an escape pod with Jennifer and Quartermain. Bruce transforms into Hulk, and manages to hold off all of the falling debris to allow the last few civilians to escape. Jennifer, scared and confused, yells out for her cousin. The Hulk simply turns and looks at her, smiling, as the entire facility caves down on top of him. Jennifer and Wanda blast off just as the underwater lab completely implodes and the entire island is shot up into the sky by a large burst of long stored up gamma energy. In their escape pod, the Samsons hold each other, and watch the explosion.
At Avengers Mansion, Jennifer, Quartermain, the Samsons, and even Betty's father gather with Tony Stark and the Maximoff twins to honor Bruce's sacrifice, as he is given a gravestone right next to Thor's. As Jennifer gives a eulogy, Pietro comforts Wanda, and even Thaddeus Ross sheds a tear.
In a mid-credits scene, A.I.M. scientists retrieve George Tarleton's body from the underwater ruins of A.I.M. Island. As the horribly mutated and deformed Tarleton is revived on an operating table, Obadiah Stane reads through a dossier labeled "Project M.O.D.O.K."
In a post-credits scene, debris from the island is seen leaving Earth's outer atmosphere and drifting off to space. Among it, the unconscious Hulk. Just as the Hulk's eyes start to open, the screen cuts to black.
"THE HULK WILL RETURN."
- Adrien Brody - Bruce Banner / The Hulk / Grey Hulk
- Fred Tatasciore - The Hulk / Grey Hulk (Voice)
- Emilia Clarke - Wanda Maximoff / Scarlet Witch
- Rosario Dawson - Jennifer Walters / She-Hulk
- Jennifer Love Hewitt - Betty Samson
- Justin Chambers - Leonard Samson
- Will Arnett - Clay Quartermain
- Jon Hamm - Phillip Sterns
- Sean Astin - George Tarleton
- Michael Biehn - Obadiah Stane
- Kit Harrington - Samuel Sterns / The Leader
- Keanu Reeves - Tony Stark / Iron Man
- Matt Lanter - Pietro Maximoff / Quicksilver
- Tommy Lee Jones - Thaddeus "Thunderbolt" Ross
- Nathan Jones - Cain Marko / Juggernaut
- Stan Lee - AIM Janitor (Cameo)
- Lou Ferrigno - AIM Soldier (Cameo)