Goliath, Quicksilver, Scarlet Witch and Vision are on an alternate Earth with Nighthawk of this Earth's hero group the Squadron Supreme. They are flying to Atomic City to stop other members of the Squadron from launching a sun probe. Last issue the Avengers saw the immediate future of this Earth, with the sun going super-nova.
As Nighthawk lands the plane the Avengers rush out to stop the launch. But of course the Squadron don't believe them. Whizzer hits the button, but Scarlet Witch's hex keeps the rocket on the ground. Then Nighthawk joins them and defuses the tension.
The probe is the creation of Brain-Child, a child super-prodigy. He developed this and other devices for the US in return for his own island, where he built his Dark Tower and hid from the world that found his form hideous.
Meanwhile Black Panther and Thor on our world have enlisted the help of Iron Man to scan other dimensions (with Tony Stark's machinery and the power of Thor's hammer) looking for their missing comrades.
When the Avengers and the Squadron arrive on his island, Brain-Child readily admits he had intended to destroy the Earth (including himself). Then he tries to kill them. The heroes split up to attack from four directions at once.
Quicksilver and Whizzer create a tornado to dispose of rocks that Brain-Child bombards them with, but then find themselves glued to a wall.
Scarlet Witch and Nighthawk meet a humanoid. It is too strong for Nighthawk, and makes Wanda forget what she is doing.
Vision and Dr Spectrum face a giant amoeba that splits into two when they attack it. And each one adapts to counter the powers of one of its foes.
Goliath and Hyperion sneak up on Brain-Child while he defeats the others. But the villain downs Hyperion with electrified cables. However Goliath uses debris to fashion a giant bow, and fires Hyperion at Brain-Child's throne, knocking him out. The Dark Tower and all the defences fade away.
Dr Spectrum uses his Power Prism to turn Brain-Child into a normal boy (presumably without his advanced intellect), and erases all memory of his misdeeds (and presumably of the reaction of people to his erstwhile mutated form). At that point the Avengers at home manage to bring the missing four back.
Vision ponders one of the paradoxes of alternate worlds. In theory there are many Earths where Brain-Child tried to ignite the sun. There may also be many Earths whose Avengers went to a Squadron Supreme Earth to save it. How do they know they have been retrieved by their own versions of Black Panther, Iron Man and Thor? They could have come back to another Earth.