Oh, it was genius, you guys. It was the It's a Wonderful Life episode, and they did such brilliant things with it. It was a big apocalyptic What If, AND, BEST OF ALL, they made it all turn out like the comic! Clark got all emo about how everyone would be better off without him ever having come to Earth, and Kara was sending him messages from the past about how Brainiac was messing things up on the day he left and instead of rushing back in time to sort it out, he said "Eh, everyone will be better off". And he found himself via dodgy sci-fi in the world where that happened, and at first everyone was -- Chloe had never got hung up on him so was all happily engaged, and Lana was in France being all married to a rich man.
And he went to the Planet and Jimmy had a bow tie like in the comics, and he had chemistry with Lois like in the comics and it was all good cheeky continuity-referencing fun, but then they did other stuff that was like the comics but scary, because Lex was President and there was a Brainiac/Luthor team and they wanted to destroy the Earth so they could repopulate it with Lex and Supergirl's children. It all made sense at the time. Oh, so very awesome, you guys.
The weird thing is I don't really care about comics-Superman that much, I mean he's nice as this big iconic figure in big DC crossovers but I have no desire to read stories that are just about him and his supporting cast. But my interest in Smallville increases exponentially with how close they get to those comics I don't care about. It's very odd. (Though it helps that they use Green Arrow as their Batman substitute, because Green Arrow's wonderful.)
And then there was Clexiness at the end, even if they were pretending to be motivated by their shared love of Lana.
There was a bit at the end when he was fixing stuff in the krypto-past where he picked up his baby self and I shouted "Blinovitch!" at the screen, but that is OK. Other fandoms are allowed to have different made up rules of time travel, really. I just secretly won't believe them.
ETA: OMG the next episode has Robert Picardo in it! <3