Spoilers for this episode.
Jack faces Kim at CTU for the first time since she believed him dead. She arrives with Barry and cannot easily accept Jack's explanation for leaving her uninformed. Their reunion is interrupted by the work still waiting for him: Henderson has been brought in for interrogation. Tony, recovering in Medical, demands to know why Michelle was killed and warns Buchanan against making a deal with the man connected to her death.
Ostroff enters CTU with McGill's stolen access card and prepares a gas canister in the ventilation system. Carrie notices an environmental fault and goes to investigate, finding the weapon before Ostroff kills her. News of Jenny's murder finally prompts McGill to admit the theft. Chloe checks the entrance records and discovers that his card has recently been used, triggering a lockdown while Henderson's condition deteriorates under questioning and Jack joins the search.
Jack kills Ostroff after misleading him over a stolen radio, but discovers the ventilation plans too late to stop the release. CTU begins evacuating as staff collapse in the corridors. Chloe seals protected rooms, and Jack directs Henderson into isolation while Kim, Audrey and the others take shelter. Edgar has gone looking for Carrie and cannot reach safety. He enters the bullpen, sees Chloe behind the sealed glass and falls dead as she watches, unable to open the room.