As the second-to-last episode of the season begins, Chuck and Sarah are trying to have a blissful day off planning their wedding. Lucky for them, Ellie’s already got her people (Awesome and Morgan) on everything. Not so lucky for them is that Chuck’s mom is breaking into Vivian Volkoff’s Colombian caves to try to steal the Norseman device, the weapon that can kill people anywhere in the world using just their DNA. And Vivian’s henchmen capture her, so Chuck, Sarah, and Casey have to head to the rescue two days before the big day. They rescue Mama Bartowski, but not without a lot of bickering between her and Sarah or a run-in with Vivian (they leave her and Riley locked up in the cell they rescue Mary from). When they get back to Castle, General Redhead fills them in on Vivian’s plan to sell the Norseman. Because Vivian knows all of Team Bartowski except one, Morgan will go undercover on the mission as a potential buyer. This is stressful for everyone, but mostly Casey, who promised Alex he’d keep Morgan safe. And we all know how difficult that can be.
At the Norseman sale, Riley and Vivian turn the Norseman on all the potential buyers, killing them instantly. Fortunately, they don’t have Morgan’s DNA, though he fakes dead sufficiently to get them to leave. When the cleanup crew comes in with acid, though, he has to use the camera glasses he’s wearing to help Casey sniper shoot all the cleanup crew through closed curtains from the next building over. Impressive! Even if it doesn’t make a lot of sense. Sarah and Chuck cut off Vivian and Riley on their way out of the building with the device. Chuck tells Vivian he’s not Agent X, like she thinks; her father is. She doesn’t believe it, until Riley confirms it, but also adds that Chuck’s mother and father made him into Alexei Volkoff to further their own CIA careers. Chuck swears it isn’t true and pleads with Vivian, but Riley sneaks up behind him. He almost kills him, but Sarah kills Riley instead, in the nick of time. Vivian’s disappeared by this point, but at least they have the Norseman. They consider going after her, but Mary reminds them they have a rehearsal dinner to get back to.
The rehearsal dinner all goes quite well, thanks to Ellie and a lovely wedding video from Chuck to Sarah (courtesy of Jeff, of all people; Lester’s creepy version was a disaster when they screened it for Ellie). Mary makes a loving toast to Sarah, so all is right between them. Then Chuck gets an encrypted phone call. It’s Vivian. She tells Chuck she’s in pain, and she wants him to feel what it’s like. Since his family is to blame for her being alone in the world, she tells him to look around and think about which one of these people she’s going to take from him. He begs her not to, but she says she’s already done it, because she has another Norseman. Sarah gets up and asks Chuck what’s wrong, and he starts toward his mom, sure Vivian’s going to hurt her. But Sarah starts hearing ringing in her ears and bleeding from her nose. She collapses just like all the others killed by the Norseman as the episode ends.