Owen is conflicted when his brother Robert asks him for help; Judd and Grace face a family crisis; As T.K. and Carlos prepare for their wedding day tragedy strikes.

Grace and Judd fight for their lives in the aftermath of the horrific car accident, as flashbacks show how a childhood tragedy led Judd to meet Grace and how they ultimately fell in love.

A recovering Owen becomes the prime suspect in the serial arsonist case. Meanwhile, T.K. and Carlos find themselves in a fiery situation and Tommy gets the surprise of her life.

Owen helps Tommy through a personal emergency.

When T.K.'s mother, Gwyn, has an emergency, Owen and T.K. travel to New York. But the flight is anything but smooth when the plane experiences a mechanical failure. Meanwhile T.K. reflects back when Gwyn helped get him into rehab years ago.

On her way back to Austin, Marjan is in for the fight of her life when she is targeted by a killer.

Grace and Carlos work together when they suspect a young woman was a victim of a black market organ trafficking ring; Owen's brother Robert comes to visit and meet T.K., but surprises Owen with news that can affect the Strand family.

Owen is conflicted when his brother Robert asks him for help; Judd and Grace face a family crisis; As T.K. and Carlos prepare for their wedding day tragedy strikes.

A bank robbery, an apartment fire and a pregnant woman lost in a parking garage are the seemingly unconnected calls to which the 126 responds. Then, T.K. celebrates his one-year anniversary of sobriety, while Owen and Gwyn may not make it to the end of the week as a couple. Also, Carlos finds himself working on a case with his Texas Ranger father.

Grace is caught in the ice storm as she goes into labor, and Judd, Owen and Tommy race to find her.

Owen helps T.K. reconcile the past to help his future; Michelle learns the full truth about Iris.

As a wildfire spreads across Texas, Evan "Buck" Buckley, Henrietta "Hen" Wilson and Eddie Diaz from the 118 firehouse in Los Angeles arrive in Austin to help Capt. Strand and the 126. As the crews race to save a group of teenagers trapped by the fire at a campground, Owen and Hen fight for their lives in the aftermath of a helicopter crash.

Captain Strand and the 126 come together when one of their own is trapped after a building explosion. Meanwhile, Owen reflects on his 9/11 experience, T.K. and Carlos come to a decision and Tommy and Julius decide on their future.

As Owen wears an FBI wire for his meeting with O'Brien, The Honor Dogs crash the meet-up and let them know there's an infiltrator in their midst. A frantic TK desperately searches for a missing and held captive Carlos.

Capt. Strand reveals to the 126 he has been working with the FBI to help bring down a domestic terrorist group with his Austin motorcycle club/gang The Honor Dogs, then race Owen to respond to a bomb threat at the State Capitol and put a stop to the terrorist deadly plans.

The crew race to a rattlesnake infestation, and attempt to rescue a man with dementia from a dangerous scene he created; Michelle learns the truth behind her sister's disappearance; A tragic accident shakes the crew to its core.

Owen and the members of the 126 race into action when a massive dust storm engulfs Austin. Season finale

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.

After a volcanic eruption wreaks havoc in Austin, the members of the 126 race to save lives at a college pool party, a family's mini-golf outing and a woman trapped in her food truck by a horde of scorpions.

Owen and the 126 race to the rescue when Catherine receives a package at the governor's office that may contain a bio-hazard. Meanwhile Grace and Carlos team up to investigate a prank 9-1-1 call that turns deadly.

Owen fears the past is back to haunt him when a stalker targets him and Catherine. Meanwhile, Tommy and Gillian face consequences when Gillian saves the life of a woman wearing a "Do Not Resuscitate" bracelet.

The 126 race into action when a major train derailment causes multiple injuries; Meanwhile Tommy makes a bold move in her relationship with Trevor; Wyatt asks Owen to give Judd his job back as Marjan and Paul continue to compete for the open position.

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.

Owen travels to Los Angeles to confront his estranged father and some painful childhood memories. Wyatt acts out and prompting Judd to take him to his family ranch where an emergency brings them closer to together.