Castle. Chuck calls on his way in to tell Sarah what's up, but he and Agent Superman have just found a chemical weapon in the base of the mask and accidentally set it off. Sarah tells him not to come in, but the weapon goes off and Castle seals the room as the computer systems say "CONTAMINATION," then a poison name that Chuck flashes on. He fills them in on the weapon and that there's a counteragent that can be taken within an hour. Chuck tells them he's going to get the counteragent from Vasillis, but then realizes he can't take the mask, which is locked in that room with Sarah and Agent Superman. He calls Casey and tells him to gear up to head back the museum. Sarah looks at Chuck, with a mixture of longing and faith. Yvonne Strahovski really is amazing at emoting without saying a word. Commercials, including one for the Heroes finale, which... don't even get me started.
Chuck and Casey, on their way to the museum, exposit that they're going to try another replica mask (apparently they just made a bunch of them, in case...). Chuck thinks it won't work, unless they can make it seem like it has the chemical in it so he gives them the counteragent. Chuck suggests gassing him, but Casey says the poison's not in this mask. Chuck says Vasillis doesn't know that. Luckily, Casey has smoke grenades in the back. His van is like a giant, weaponry version of your mother's purse: Everything you might possibly need is in there.
Agent Superman, all sweaty, doesn't feel like he's dying. Sarah, sweaty and looking like she might pass out, says Chuck will come through for them. Agent Superman wants to clear the air in case he doesn't: He hasn't been completely honest about the coffee or how he was acting at the museum. He was hitting on her, which he apologizes was completely unprofessional. She apologizes, too, for overreacting, and says the way he touched her neck at the party was actually kind of nice. They're both glad they had this talk. And then the contamination is contained. Agent Superman realizes -- based on her passing out -- Sarah needs the antidote and shoots a hole in the glass and carries her out. Back at the museum, Chuck enters and Vasillis and his men stop him, all: "Show me my mask." Chuck pulls it out of his bag and says, just as threateningly (okay, maybe not), "Show me my Nerd Herder." Vasillis says she's in the vault and the air must be getting pretty thin. Casey's somehow already above the vault, and is ready to blow it if he doesn't open the vault. Chuck says to let her out since he did what they asked. Vasillis says he would, but the people Chuck stole from, the Ring, they're not forgiving. Chuck tells Vasillis to give him the girl, but Vasillis says he gave us any leverage he had by walking in with the mask. Chuck says that's not quite true and holds up the mask, then gives them one last chance to open the door and let her out. Vasillis doesn't believe him, so Chuck drops the mask, and Vasillis says he's killed them all, because all he knows is the counteragent's in one of the hundreds of vases in the museum. Vasillis starts breaking them individually, but Chuck flashes on a stolen one and then hits Vasillis over the head with it, knocking him out and revealing the counteragent. Casey prepares to blow up the vault, but Chuck stops him after he gets the counteragent. Casey: "When do I get to blow something up?"