Brainiac 5 finishes a time viewer, Supergirl (of that time) uses it to look in on Superman. But while doing so, they go through 4 or 5 versions of Superman before getting to the one of the new continuity. Golden Age, Silver Age, Mullet Age, and Birthright. Brainy and Kara look at his past with retcons involving:
Clark's adventures with the Legion
Clark not in civvies when he saved the space plane
Lex in Smallville
Martha's dad's funeral
Donner and Johns' interpretation of Krypton
Superman and the Justice League
And some Loeb stuff including Superman/Batman
Spoiled!
