Giant-Size Invaders #1 (there was no #2) was used to launch a new Invaders series, which seems like a good use for a GS special. Sadly, the comic isn’t very good. The cover to the first issue of the series was good, though.
It’s WWII adventures. You pretty much know from the start what it will be like.
It starts in 1941, with the team flying over London in one of Namor’s Atlantis airplanes and fighting Nazi bombers. Bucky and Cap go on the ground to help civilians, where Cap saves a woman with amnesia from getting injured, but while doing so she drops a ring. Captain America literally rationalizes stealing her ring. Look at those panels. “I’ll just hold on to it for safekeeping…” Wow.
Then the team goes to look for a villain named (seriously) Brain Drain. Brain Drain is literally a brain in a robot body, but the dude is dumb enough to make the dome clear, so you can tell he’s just a brain. Moreover, the brain looks more like a liver. Anyway, B.D. captures Captain America and the amnesiac girl, named Hildy, who for some reason Cap is still carrying around with him. Good thing, too, because it turns out she’s an alien who Brain Drain has been tracking, and the ring was the Ring of the Nebulas. (Not NebulaE, NebulaS.) She destroys it and then kills herself. Brain Drain then says he loves her and he also kills himself.