Chloe and Jimmy are hanging out, one week before their wedding, when our bride-to-be begins to lose memories of her engagement party. As we look at the memory of that night, objects in the room are being replaced by weird computer code until finally, the man she was dancing with, Jimmy, disappears, too. Chloe comes back from the memory and can't remember Jimmy at all. Let the Amnesialympics begin! Jimmy goes to Clark for help. Chloe remembers him, but can't conceal the fact that she's forgetting everyone else in her life and has to keep a meticulous Wall of Chloe's Weird Life at the As-Is Foundation just to keep everybody's names straight.
Davis, meanwhile, is looking for help from Chloe, but keeps getting the brush-off. He gets the sense that something's wrong with Chloe and tries to help her. He also tells Jimmy, who is very freaked out by Chloe's memory loss, that he won't be able to attend the wedding.
Clark decides to use the blue crystal to rebuild the Fortress of Solitude, even though using it last time sent him to the Phantom Zone. He's convinced it's the only way to save Chloe's memory, which they realize is being taken over by computer code from Brainiac. Everything is being replaced by the Kryptonian symbol for "Doom." Before Clark can get to the Fortress, Chloe's memory of Clark's powers is erased. Clark heads out to the Fortress, which is implausibly rebuilt this time with no problems. Jarnelle and Clark discuss matters of Chloe and Brainiac, and Jarnelle is impressed that Clark is being so mature about all this. Clark is impressed that Jarnelle was willing to give up his own son to save Earth. Aw, go play Kryptonian catch, you two. Clark wants to restore Chloe's memory and get rid of Brainiac. Jarnelle warns him that there may be grave consequences.
Jimmy, alarmed that Clark left Chloe alone at the barn, takes her to the hospital. An MRI shorts out the machine and Chloe loses one of her most precious memories -- a kiss she gave Clark when they were both much younger. She drags Clark through several memories, a la Eternal Sunshine of the Spotless Mind but eventually loses him, too. Only one person remains in her brain: Davis. Chloe, terrified, finds Davis, and he takes her back to Smallville in hopes that seeing Clark and Jimmy will jog her memory. It doesn't, so Davis sedates her with a shot. While Chloe is unconscious, Clark zips her out of there and to the Fortress, where Jarnelle is able to bring Chloe back to normal. Clark asks him to take away any of her memories of Clark's powers or anything Kryptonian in order to rid himself of the burden of feeling guilty about Chloe keeping his secret. Even after she'd told him she wouldn't trade that for anything. It's complete bullshit and a violation of their friendship, but Clark is too stupid to realize that. So the mindwipe continues and Chloe now has no recollection of her years working with Clark (and Oliver for that matter). Way to fuck your best friend over, Clark. Chloe goes back to being oblivious and planning her wedding with Jimmy. But there's one hitch: Davis wishes he'd never brought her back to Jimmy and Clark and professes his love for her. He says he'll wait as long as it takes and won't ever let her go. He kisses her and Chloe cuts it off before it gets too hot or too heavy.
Clark goes back to the Fortress and thanks Jarnelle for saving Chloe. He even calls him "Father," which is nice, except that right after Clark exits, Jarnelle calls for his son as black goo that came out of Chloe's ear begins to infect the Ice Dildos and the entire Fortress. Apparently, Brainiac is not gone completely and he has a warning for us: "Doomsday is coming!" he says.