The Avengers are attending a ceremony in New York's Central Park to honour their defeat of the Mandarin's plan to take over the world in Annual #1. This day has been declared Avenger Day. Iron Man and Thor, who participated in that adventure, accompany the current Avengers, as does their guest Hercules who is considering returning to Olympus. Quicksilver and Scarlet Witch are there, although Pietro has revived his old resentment at anti-mutant distrust. Hawkeye is there, having just come from Black Widow's hospital bedside (injured in #44 in the battle against her ex-husband Red Guardian), where Natasha has given up her costumed identity and Hawkeye has effectively proposed to her (but her reply was a bit evasive, and I don't think anything comes of the proposal). Goliath is there but without the Wasp, who turns up fashionably late to show off her new costume and sports car. The public and press know that Wasp is Janet van Dyne, who has recently inherited 5 million dollars.
The only Avenger missing is Captain America. Various Avengers perform stunts to keep the crowd amused. But eventually Thor and Iron Man have to leave on private business. Soon after that Cap arrives, without saying why he's late, but thinking about giving up his costumed identity too (which will come to a head in Tales of Suspense #95). Now the ceremony proper takes place, culminating in the current Avengers making Hercules an official member.
But hidden among the crowd is the Super-Adaptoid, an android built by AIM, able to absorb the abilities and powers of others. It was sent against Cap in ToS#82-84, where it absorbed the powers of Cap, Goliath, Hawkeye and Wasp, who were the Avengers at the time. The battle only ended when the Adaptoid thought it had killed Cap. But SHIELD had destroyed AIM in Strange Tales #149, and the Adaptoid was left purposeless until it learned that Cap was still alive. It has now added the powers of Pietro and Wanda to its arsenal, as well as Iron Man's repulsor rays and the might of Thor and Hercules, and its shield has some of the power of Thor's hammer Mjolnir. Now that its target has shown up it sheds its human disguise and attacks.
Cap spots the Adaptoid and intercepts its first blast. The other Avengers then try to keep it away from Cap. Adaptoid grows to giant size, so Goliath takes it on first. Then they both shrink, and the battle continues at ant size, including the Wasp. But Adaptoid KO's 'Ant-Man' with its combined powers. Even when Wasp grows to human size with Adaptoid held between her fingers she is unable to crush it.
Hawkeye's arrows fare little better, until Cap takes on the Adaptoid himself. Adaptoid casts a hex to keep the other Avengers away, and all too soon has its target at its mercy. But its attention has wavered and so its hex fails. Hercules steps in to save Cap, followed by Scarlet Witch, and then Quicksilver.
Pietro has noticed that the Adaptoid only uses a couple of powers at a time, and devises a plan. Although the enemy has by now knocked him out, the other Avengers implement the plan. By ganging up on it they encourage the adaptoid to try to use all its stolen powers at once. And it overloads its circuits and collapses.