Two SUVs crash. One overturns. An off-duty paramedic named Eileen arrives, sees a girl trapped inside and a man under the car. Eileen enlists a bystander to help rescue the girl. Seconds later, the car explodes.
An assistant D.A. brings three joy-riders who fled the scene into the Lightman Group office. Gillian tells Ria that the rescued girl is recuperating, but the man - pro baseball player Juan Salazar - died instantly. The D.A. wants Cal's team to determine which of the three joy-riders was the driver, who'll be charged with second-degree murder. As they watch video taken at the scene, Cal arrives and notices guilt on the paramedic Eileen's face.
Ria and Eli study the guys in the cube for a minute, then send two away. Ria tells the third, Mark, that their body language said they knew he was driving - and so does she. But Mark's lawyer interrupts and takes him away.
Cal finds Eileen at the hospital and asks about her guilty look on the video. Eileen says that she was just doing her job. On her way to the parking lot, Cal accuses her of being an adrenaline junkie who gets her kicks from saving lives. A man appears with a ball. Eileen says that he's her brother, Kent, then shoos him away and calls Cal crazy.
Back at the office, Gillian and Ria tell Mark's mother that they believe the D.A. is over-reaching. When she mentions that Mark told her the light was green, they promise to check that out.
Cal is driving while talking to Emily; he sees his light turn green, proceeds, and gets broadsided. Cal exits his car, bleeding, and collapses. An out-of-uniform Eileen arrives and gets Cal to the hospital. Eileen tells Gillian and Emily that he's got a concussion, there's a 24-hour hold, and she was just driving home from work when she saw the accident. Cal doesn't buy it. He tells Eileen to get lost and voices his suspicions to Gillian, who reports that the other driver claims that Cal ran a red light.
Eileen goes to the office and tells Ria to drop any further investigation. If Mark didn't run a red light, he shouldn't have to do hard time, replies Ria, noting the shame on Eileen's face.
Cal climbs out of the hospital bed, heads for Eileen's locker, and discovers a scrapbook with 16 newspaper clippings about Eileen's rescues - all from the same time of year. When Eileen storms in, Cal notes that Salazar was the first person she didn't save - but he died on impact. She's angry, but also terrified, and walks out of the hospital. Meanwhile, Ria and Eli visit Mark in jail and determine that his claim about not running a red light is true.
Cal awakens at home, finds Eli, Gillian, and Ria, and they compare notes. Eli mentions that the secret service and FBI can change lights via radio transmitters on their rear-view mirrors. Ambulance drivers have similar handheld devices, says Cal. Eli adds that Cal missed the news clip about Eileen's mother dying in a car crash, making her and Kent orphans. Eli also notes that the clips are from every town they've lived; Eileen was always first on the scene, and she was always off-duty.
Cal and Gillian visit Eileen's apartment and ask Kent, clutching a ball, about their mother's car crash. An accident, he says, adding that Eileen was a child, bouncing a ball off the dashboard when it happened. He breaks down and refuses to talk. Gillian suspects that Kent has neurological damage.
Reviewing cellphone footage of Cal's accident at the office, Gillian notes that Cal's light was green - but there's no ambulance in sight - and a fearful Eileen looks like she's expecting someone. Cal spots Kent in the crowd, and Eli points out the joy on his face.
Cal enters Eileen and Kent's apartment, finds a collection of jars filled with shattered glass and several handheld signal-changing devices. He pockets one, goes outside, and Cal shows Gillian how it works, noting that it was cloned from Eileen's ambulance issue.
Meanwhile, Ria shows the assistant D.A. a video of Kent at the Salazar crash scene. She doesn't want to know. Eli tells her about all the previous accidents and how prosecuting those cases could help her career - and save lives.
When Eileen returns home, Cal confronts her with the jars and devices, adding that she could've stopped Kent years ago if she hadn't felt guilty for her mother's death, but he is interrupted by Eileen's getting a message about a nearby accident. Cal and Eileen race over, discover Kent's been hit by a car. The device he used to control the signal is beside him. He wants to hurt Eileen by having her watch him die, says Cal, who pries the ball from Kent's hand, and orders Eileen to save him, which she does. Cal hands Eileen the ball, which she hurls away.
Cal visits Eileen in prison, noting that she never wanted to hurt anyone. Eileen asks what Cal's found. "A life," he says. "Worth saving?" she asks. "Look after yourself," answers Cal, leaving her smiling.
Back at the office, Ria tells Mark's mother that the D.A. is reducing his charges. She thanks Ria for not giving up. "Just doing my job," replies Ria.
Cal is home with Emily, who has found a scrapbook he'd hidden from her. She asks him about some embarrassing photos. He lies. She knows it. Cal says she gets her mouth from him and don't forget it.