The Leader has placed an expanding dome of pure gamma radiation around the Cube, with anyone caught inside it being mutated into a Gamma Monster.
Introducing characters: Madame Hydra
Synopsis for "Gamma World, Part One"
The Avengers, sans Ant Man, pursue Hawkeye and finally manage to corner him but they receive a summons from S.H.I.E.L.D., allowing him to escape in the confusion. A gigantic green dome has appeared around the Cube that is composed of pure gamma radiation. The doomed Hulkbuster squad was taken out by Emil Blonsky, the Abomination, and an unlucky team of S.H.I.E.L.D.agents were caught inside the dome when it appeared. Security footage shows them mutating horribly after their exposure to the gamma radiation, a fate that will befall everyone present soon enough as the dome is expanding; it is projected to hit a major population centre in 6 hours. SHIELD requests that the Avengers go into the Cube, accompanied by a handful of SHIELD agents and a much-recovered and significantly more ripped Doc Samson.
We then cut to Hawkeye who is stalking Black Widow all the way to a Hydra safe house where he proceeds to kick copious amounts of ass and ends up taking her prisoner. He can't take her back to S.H.I.E.L.D., since he's still officially a Hydra double agent but he can take her to the Avengers.
Unfortunately the Avengers are having their own problems as they have to square off against the horrors unleashed by the gamma radiation dome. First they encounter the mutated S.H.I.E.L.D.agents, whom Samson believes can be returned to human form once they leave the irradiated area, and then a laundry list of gamma-powered villains under the command of Samuel Sterns, the Leader. In addition to their numbers, the villains also have their powers enhanced by devices that were originally produced by Samson to drain gamma energy but have been modified to do the exact opposite. Although the heroes come out on top, it was all a ruse to breach their gamma protection suits, as they find out to their horror when the Wasp begins to mutate as well.
The Leader gets his allotted evil gloating time for the episode before telepathically commanding his creations (including the Avengers' S.H.I.E.L.D. backup) to deal with the intruders. Black Panther breaks away from the group and leaves them to fend for themselves, but they soon realize that he has the right idea and beat a hasty retreat towards the generator for the dome. Thor and Samson break through the monsters and gamma-powered supervillains, only to find the device guarded by the Wrecking Crew. Thor is no match for them, now that they are enhanced, but he keeps them occupied long enough for Panther to destroy the generator, allowing S.H.I.E.L.D. to round up the escapees and turn everyone back to normal. So once again, the day is Saved, thanks to the Avengers.
The Leader, Abomination and the Absorbing man look at Las Vegas in the distance and activate the Gamma Device having completed their test run at the cube.