Castle. Agent Superman brings Sarah a coffee, and she says they need to talk about professional boundaries: him bringing her coffee every morning and noticing she chews on her swizzle stick. She says he's embarrassing them both, since they're just colleagues. She asks him to keep his coffee and cheesy come-ons to himself. Agent Superman feels like he should probably apologize to Colonel Casey, who walks in then and thanks Agent Superman for the coffee, just the way he likes it: black and bitter. Sarah looks a little sheepish as Agent Superman looks hot and smiles. Hannah shows up at the museum and Vasillis thanks her for coming. He tells her the curator's tied up and he's the assistant. He wants her to go to the vault, but makes sure she doesn't have her phone first. He sends her into the vault and, of course, locks it. Right away, it starts emptying of oxygen. Vasillis calls Chuck and tells him to bring the mask or he kills his girlfriend. Chuck asks Morgan if he's seen Hannah, and he says she's on a call at the museum. Chuck tells Vasillis he doesn't have a girlfriend, and this is the Nerd Herd desk, but Vasillis basically tells him to cut the crap, and he'll bring the mask in 30 minutes if he wants Hannah to live. Morgan tells Chuck they need to talk, but Chuck runs off; no time. Morgan tries to follow, but gets all tangled in the phone cord and misses Chuck going through the hole in the floor of the display room. So close.
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.