Synopsis
In #26 the Avengers responded to a call from the Wasp to stop subsea villain Attuma flooding the surface world. In #27 they foiled his plan, but couldn't find Wasp. Now they get a call from Henry Pym saying Wasp escaped Attuma, reached Avengers Mansion (at least according to Wasp's later version), and contacted Pym to say she was on her way back to their scientific research vessel. But he hasn't heard from her since. He wants them to come and pick him up in an Avengers rocket-jet air car.
In order to get them to do what he says he reveals his identity as Giant-Man. The Avengers didn't know this, not even Captain America, because the original team kept their identities secret from each other. Hawkeye goes and fetches Pym, but the Avengers still aren't sure he's who he says he is. They want him to prove it by changing size.
Pym says one of the reasons he left the Avengers was because size-changing was affecting his body. But he'll risk it for Wasp's sake. Scarlet Witch gives him a costume she had conveniently sewed for him using stretch fabric he left at the mansion. He tells them he doesn't need a potion to change size anymore, he can do it mentally. But he can no longer shrink smaller, he can only grow. And only to one size - 25 feet. Moreover he can only stay at that size for exactly 15 minutes, no more, no less. So saying he gains height. And they rechristen him Goliath.
Meanwhile they receive a transmission from a new baddie called the Collector, who tells them he has Wasp, and where they must go to rescue her. Collector, as his name implies, collects things, including people. He transported Wasp from Avengers Mansion to his castle. He has also collected the Beetle, a minor villain with no particular attachment to any one hero, and dosed him with an obedience potion. Collector now wants to complete his collection of Avengers. Presumably that is why Beetle attacked Hawkeye last issue. And I guess Collector retrieved him from Hawkeye's trap.
There is a question over why Collector needs to lure the Avengers to him, or send Beetle after one of them, when he was able to grab Wasp from the mansion. In some later appearances he is able to teleport full-size people. Perhaps his teleportation equipment is experimental, and he can only use it on small things at the moment. But if so then maybe he didn't rescue Beetle, and Beetle escaped by himself.
The Avengers including Goliath arrive at Collector's castle. Goliath is back at man-size because his 15 minutes were up. But Collector captures them all with sleep gas, and chains them up. He doesn't recognise Goliath, and is upset because he wants Giant-Man to make a pair with the Wasp. For some reason he considers the four current Avengers plus Giant-Man and Wasp would constitute a full set, and is not interested in Iron Man or Thor. In some later appearances he is credited with limited precognition. Maybe he foresees that Goliath and Wasp will soon rejoin the Avengers. But it still doesn't extend to recognising Goliath's new costume.
Goliath breaks free by growing, and releases the others. Collector flees. Goliath is now stuck at 25 feet for 15 minutes, and finds it difficult to move about in the confines of the castle. The others leave him to go look for Wasp. Quicksilver runs ahead, but the other three are delayed by a fight with Beetle, with his armoured wings and flexible suction-cupped 'fingers'. The villain traps Hawkeye and Wanda behind a rockfall, and knocks Cap out.
Pietro and Hank happen to find Collector at the same time. He fights them with weapons from past ages, eventually rendering Quicksilver unconscious and pitting Goliath against two giants. Beetle arrives with captive Cap, but closely followed by Hawkeye and Wanda after the Witch freed them from the rocks. The 2 conscious Avengers help Goliath defeat the giants.
Collector has Wasp imprisoned in a vial she can't grow out of, and he holds the heroes at bay. But Cap and Pietro have recovered, and Quicksilver relieves him of the vial. But Collector uses a temporal assimilator to whisk himself and Beetle to elsewhen.
Wanda shatters the vial, and Wasp grows to woman-size. She follows Hank's example by revealing her identity as Janet van Dyne. But when Goliath tries to shrink he finds he has been a giant too long, and collapses unconscious at 10 feet.