The 126 race into action when an armored truck and its guards are attacked by a group of masked men. Carlos, in his new job as a Texas Ranger, is assigned to investigate the case. Owen attends group therapy with Tommy to deal with his brother Robert's death; Both Marjan and Paul each apply for the open lieutenant position, replacing Judd, who finds himself lost without his old job. Wyatt begins work at the call center and a major train derailment signals tragedy.

The 126 race to contain a toxic chlorine cloud of death unleashed by the train derailment. Owen names the station's new lieutenant as Judd finds a path forward.

The 126 are called on the scene to a hoarder's rat-infested house, then are trapped in an elevator as a woman goes into labor. Tommy and Trevor host his ex-wife Cassandra for dinner which goes awry.

The 126 race into action when a horse barrels into a brewery with an unconscious rider on its back and then to a bus full of tourists all experiencing seizures. Judd introduces Owen to Wyatt's mother Marlene. Carlos and T.K. start couples counseling, when Carlos investigation into his father's murder becomes all-consuming.

The 126 race to save a man crushed under a vending machine. While struggling to handle his aggressive foster horse, Owen finally faces the truth of his brother's death. As Mateo and Nancy scramble to delete a nude photo of her accidentally sent to the 126, Tommy is concerned that Nancy may be facing a possible health scare.

TK gets a huge surprise on his 30th birthday when his stepdad Enzo arrives in Austin along with TK's young half-brother Jonah; Owen's resentment of Enzo turns physical; Tommy is determined to push through her latest crisis; Wyatt attempts to talk-down a suicidal caller and the 126 must perform a dangerous surgery in the field after a man is impaled in a tree.

A gang killing leads Carlos to new information on his father's murder. TK contemplates a major life decision as Owen is determined to get Austin's first responders' health care coverages increased.

Carlos is determined to solve his father's murder. Owen is offered a life-changing opportunity. Judd gives into his demons as Tommy begins her treatment.

Judd literally walks into the line of fire when an armory catches on fire, sending bullets flying everywhere. Owen questions Judd's pledge of sobriety. Marjan introduces boyfriend Joe to her parents, Waleed and Nasreen, but their dinner doesn't go as smoothly as she hoped.

Owen makes a decision about his future. Tommy receives devastating news. T.K. and Carlos worry about adopting Jonah. Mateo gets his immigration status threatened after an altercation. A doomsday prepper gets trapped in a bomb shelter and The 126 and all of Austin brace for an asteroid crash.

Owen and the 126 deal with the aftermath of the asteroid crash in Austin while an even greater threat looms.