Two men are found dead in Seattle, one killed by a car and the other strangled and left on the road.
The only similarity to each other - and the reason the BAU are called in - is that both men have mouth mutilation. Plus their cars are found about two miles from each other.
Meanwhile, against Morgan's objections and advice, Garcia takes some time off to go visit Greg Baylor, a prisoner awaiting execution on death row. He's the man who she shot in defense of Reid and herself in the hospital on Criminal Minds Season 9 Episode 24.
As the team investigates, two more murders take place - one by drowning and the other is burned to death in a wooden box made to look like a coffin.
The BAU discovers that on the roof of each victim's mouth is a carved Roman numeral. They determine that the deaths are crude reenactments of the Nine Circles of Hell, found in Dante's Inferno.
The Unsub, whose father had abused him and his brother, had been picking parental victims, and killing them in proxy revenge.
The BAU determines that the Unsub is a teacher - named Justin Leu - who had impressed one of his students - Nathan Chow - with Dante's inferno. The troubled student had written a blog about killing his classmates, and had been preparing himself by obtaining explosives.
The team figures out that Chow may be the Unsub's proxy Virgil, so Morgan and Hotch go to his dorm.
The dorm is empty, but they find the boy's explosives. Chow arrives at his dorm, and they arrest him.
The BAU tracks Leu to his father's place, and catch him just as he's about to kill his next victim by burying him alive in a grave. Leu points the gun at his victim, but they talk him into lowering it. They arrest him.
Meanwhile, Garcia meets with Baylor in prison. He asks her to be with him when he's executed. She at first refuses, but then relents and shows up for it.
Garcia arrives home to find Derek Morgan waiting for her on her front steps.