The team races to find a group of kidnappers who have their victims drain their bank accounts using ATMs, and Charlie's attempt to reinstate his clearance is slowed by an agent with an agenda that may include Don.
Charlie and Don's fates hang in the balance as the team tracks a white-collar criminal through Los Angeles. Meanwhile, Alan finds out that one of his old planning projects may disappear with a new development by the city.
The death of an agent kicks everyone into high gear, but that slows down when their prime suspect turns out to have Autism Spectrum Disorder, Charlie steps on toes re-establishing himself as the resident expert, and Don looks for deeper meaning.
Two trains collide, a passenger train and a freight train carrying toxic chemicals. Don, Charlie and the team are brought to the scene, investigating a potential crime.
The team descends into a bit of madness as working with a conspiracy theorist to solve bombing case exposes each of their own peculiar beliefs. Meanwhile, a mysterious stranger saves an agent's life.
Buck Winters, who murdered and stole with his older lover in the season three premiere, escapes from prison and comes after Don. Don uses his newfound faith to guide him through.
Charlie's hacking skills are put to the test when the team tries to rescue hostages on a bus. Meanwhile, Alan chafes at his first experience with the think tank model.
When Robin Brooks loses a case involving the murder of an informant, the team takes it very personally. Launching an investigation to get around double jeopardy, and bring justice down on an international arms dealer.
An old friend who's a DEA agent agrees to assist Liz in an undercover operation when Charlie uses math to affect the city's drug traffic, but a miscalculation and an addiction endangers both her life and the operation.
When an AI kills the scientist who created her it opens a career door for Charlie, but first he and Amita must determine what happened, putting Amita's life in jeopardy.
Charlie is distracted from applying math to his first basketball-coaching job when a doctor claims to have evidence that clears a death-row inmate, leaving him and the team very little time to be sure real justice occurs.
Charlie once again questions his involvement with the FBI when a miscalculation leaves Don in the hospital and his team searching for the reason behind atypical home invasion robberies.
Charlie, still feeling guilty about Don's injury, focuses on finding a frighteningly intelligent serial killer who is meticulous about eliminating all witnesses to his crimes.
Disgraced agent Roger Bloom appears to be involved in a string of bank robberies, Charlie resists becoming part of his new office's legacy, and an abduction will forever change the Eppes's lives.
Wracked by fear and self-doubt after Amita's abduction, Charlie must somehow focus when she reaches out to him in order to save her before it's too late.
