A man begins digging a grave for a dead body.

Later, the man - Charlie Senarak - is driving along a Hollywood highway when his wife Tracy calls him because he's late for work. They argue about the fact she cheated on him with a man named Theo Koutranis.

A bus driver loses consciousness and crashes. All the people on the bus have died from sarin poisoning. There are ten confirmed dead in all.

The BAU is called in to investigate.

Charlie arrives at work and receives a text which says "I know what you did".

The person at the other end is blackmailing him. 

Charlie receives a cellphone in his office mail, only he doesn't know it's his. The blackmailer calls him and tells him to break into an office and steal a laptop.

The BAU discovers that one of the bus passengers, named Michael Hoffman, was a freelance journalist who worked in Syria.

Investigators find a time-release mechanism on the bus that was used to distribute the sarin at a specific time.

In her investigation of chemists who are feeling pressured, Garcia hits on one named Johan Geitman who was fired 2 months ago for online gambling. He subsequently had posted a rant on social media threatening the company.

Suspicion turns to Charlie after Agent Colfax pulls camera footage showing him spending only a few minutes at a bus station, then cleaning his clothes and shoes afterward.

Rossi and Lewis interview him. They know he's lying about something but not about the sarin attack.

Charlie arrives at a cabin and takes the laptop as per the blackmailer's instructions, only to discover a dead body at the scene. Just as he goes to leave, police show up.

Police identify the dead man as Johan Geitman.

The BAU believes Geitman provided the sarin for the attack.

A flashback shows that Charlie killed his wife's lover, and that the blackmailer knows this.

The blackmailers tells him to go to a warehouse and pick up a package for him.

Police find camera footage that places Charlie's car near the scene of Geitman's murder.

Garcia finds that Charlie has been paying money for marriage counseling, despite his earlier assurance that his marriage was fine.

Garcia uncovers evidence that Charlie has been in a rage about his wife's infidelity.

Rossi and Reid arrive at his office, only to find he had left a half hour earlier.

Charlie arrives at the warehouse to pick up the package. His name is on the package. He opens it and finds a gas mask and hazmat suit inside.

The unsub is preparing several containers of sarin.

Reid and Rossi re-interview Charlie's wife Tracy, who tells them her affair with Theo wasn't quite over, but she's been trying to end it. She tells them Charlie thought it was over.

The unsub calls Charlie and they argue. The unsub mentions Charlie's daughter. Charlie, believing his daughter is in danger, rushes back to the FBI.

Garcia informs the team that Theo Koutranis' (Tracy's lover) real last name is Martin, and he's part of a cult called Heaven's Mission.  Theo has a brother named Mitchell Crossford.

Reid believes the brothers were planning a terrorist attack, but Theo backed out.

The BAU realize Mitchell has been manipulating Charlie. They theorize, as per Mitchell's online rants, that he's targeting the FBI.

Charlie shows up at the FBI building. Hotch opens the trunk of Charlie's car and discover the time-delayed sarin containers.

The team realizes that the time delay is probably set for 5 minutes later, when everyone is getting ready to leave for the day.

They discover a van belonging to Mitchell, but he's not in it.

Garcia tracks his movement within the building.

Mitchell takes out a gas mask. Hotch silently comes up behind him and arrests him.