The cover copy says "Caution: This may not be the best story you've ever read... but with a cover like this, how bad could it be?!?" Pretty bad, actually. It hinges on a story I've never heard of that already takes place in an alternate timeline where apparently, the turning point is "What if the Avengers were gullible morons?", and goes from there. I'm trying to get my head around it without reading the original story, so to better explain it, lets call the Avengers of this tale those of Earth-689, while our own timeline is Earth-616. Hopefully, that'll help.
What If Vol.1 #29 (October 1981)
Based on: Avengers Annual #2 (1968)
The true history: The time-traveling Scarlet Centurion goes back in time to make the original Avengers (Hulk and all) an offer - capture all the superhumans on Earth for him to neutralize as it's the only way to prevent the apocalypse to come. They agree without so much as a letter of recommendation, round up everyone but themselves, which the Centurion puts in his collection. Immortus (one of the Centurion's other selves, he's also been Nathanial Richards, Rama-Tut and Kang), trying to undo his own handiwork, then transports the Avengers of Earth-616 to Earth-689 to defeat the last five heroes remaining, They do so, then turn on the Centurion and defeat him before being returned to 616.
Turning point: What if the Avengers of Earth-616 hadn't been sent to Earth-689?
Story type: Everybody dies
Watcher's mood: Chicken poxed
Altered history: Earth-689 deviates into yet another timeline when the "real" Avengers fail to materialize. This creates two changes. 1) No one can ask question as to how Captain America, Hawkeye and Black Panther were a match for the original Avengers who just defeated every superhuman on Earth; and 2) pop will eat itself. They force the Hulk to turn back into Bruce Banner and hand him over to the Centurion (the tribe has spoken) and then quit the superhero life for good. Thor goes back to Asgard, Tony Stark to girls and booze, and Hank and Janet to their marriage.
(To see the Centurion's action figure collection up close, click to boost with Pym particles.) But Thor keeps an eye on Midgard-689b, and when the Centurion tries to take over the world, none of them can stick to their New Year's resolution.
So off they go to fight the Centurion, primed to take over the world what with only four superheroes to stop him. They are outmatched by his fearsome future technology.
Except Thor, who beats the crap out of him. Should the Avengers come out of retirement? Nah. Nobody will trust them now that they've taken out everybody else. So they don't. And that, my friends, is what we call going out with a whimper.
Books canceled as a result: None, because there are no Earth-689 comics.
These things happen: Well, Earth-689 does exist, and the same story was later retold using the Squadron Supreme on yet another Earth.
Next week: What If the Sub-Mariner Never Regained his Memory?
My guess: Never could find his keys.