Casey and Sarah are arriving at the opera house, where we see what I presume is the fat lady from the episode title. She's practicing her aria in front of an empty house, but in full tacky makeup. Maybe she goes on soon? I don't get it. Sarah asks Casey whether he's concerned involving a "civilian" in a covert government operation. Casey says her knowledge of Gui makes her useful, and he thinks Sarah's just upset that Chuck likes her. Chuck and Jill are up in one of the boxes (the one where Gui sat) looking for a lock. Chuck finds it in the floor, completely obvious and shiny. He opens it and they pull out another box. The four of them meet on the stage to regroup and open the box. Just then, our mystery guy pulls up (I'll assume it's outside the opera) in his van. Back inside, they open the box, which starts counting down from 1:06, and Sarah tells Casey it's a bomb. She must have done so well in her bomb-countdown-clock detecting class at spy college.
Jill says it's not a bomb, it's a clue, because Gui wanted to help people, not hurt them. Sarah thinks she's wrong and it's too dangerous, but Jill says smugly that she's never wrong. They try to solve the puzzle, which is a sheet of music, as Casey evacuates the building. They solve the puzzle, with the bad Fulcrum guy looking on. Chuck's excited, and he and Jill kiss. Sarah can't stand it and turns the other way. Casey says he loves love, and Sarah looks at him and rolls her eyes. I like that Casey speaks up about all of this, and knows exactly where Sarah stands with Chuck, feelings-wise. He might not care about feelings, but he sure does know about them.
Back at OOSSL, Agent Redhead says the computer is trying to find an encryption key to figure out the data. She wants them to thank Chuck for his service, and wonders where he is. Casey guesses he'd be rounding second base. Sarah looks upset. Back at Chuck's, Jill and Chuck are getting hot and heavy on his bed, when he stops it, because he can't do it in front of the camera. He tells her to meet him at the Buy More tomorrow. She asks what time, and he says (I have to quote this directly): "Take the latent heat of aquatic fusion ... and subtract the atomic number of thulium." He asks if she's got it, and she does. We get a shot of Casey listening in. He says, "Nerd code," and appears to be stumped. But, um, the meeting is tomorrow, Casey. Look those things up. You have infinite resources at your disposal, I'd think. Or you can just start with Google.