On one of the hottest days of the year, the first responders feel the pressure as harrowing incidents keep happening around the city, including a road rage accident involving a van full of tourists, an emergency at an auto-repair shop and a veteran with a live grenade embedded in his leg. Meanwhile, Buck gets a surprise visit from his sister, Maddie. Also, Athena questions her relationship with Bobby. Then, at the fire station, handsome new hire Eddie jeopardizes Buck's chances of being chosen for a fireman calendar.
When a massive earthquake rocks Los Angeles, the first responders rush to rescue victims from a crumbling high-rise hotel both under the rubble and high above the ground. Athena must deal with wreckage from a collapsed freeway overpass and Maddie is thrown into the fire as she faces her first crisis as a 911 dispatcher.
The first responders continue to deal with the fallout of a massive earthquake and its deadly aftershocks. Athena tries to keep the peace as Bobby and team continue to rescue victims from a collapsing high-rise hotel, both under the rubble and high above the ground. Maddie's first day on the job involves helping a pregnant couple deliver their baby safely.
Wanting to see the faces behind the 9-1-1 calls, Maddie goes on a ride-along with Athena, while dealing with a seemingly "perfect" dispatcher back at the call center. Meanwhile, Hen must figure out how to save her family from permanently falling apart, and the first responders deal with emergencies both at a military funeral and a movie theater.
The first responders race to rescue victims trapped in various tight spots. Meanwhile, Athena contemplates accepting a promotion that will take her out of the field, and Maddie decides to make a move of her own. Then, Buck wonders if he should move on, and Chimney finally deals with the aftermath of his near fatal car crash. Also, Eddie turns to the crew for help with his young son.
When the first responders race to the rescue of a crashed news copter, they unwittingly become the subject of an eager reporter's first on-air assignment. Meanwhile, Bobby tries to deal with his daughter's death, as Athena prepares for May's homecoming dance. Then, Maddie struggles to let go of her fear of the past. Also, the team responds to emergencies at an eating contest, a bodybuilding competition and a toddler pageant.
Halloween finds the first responders racing to the rescue at a cemetery, a haunted hayride and a spooky Hollywood parade; Meanwhile Maddie thinks she has received a "ghost" call from a missing hiker; Hen finds herself facing with a life or death situation with the father who abandoned her; Eddie reconnects with his estranged wife.
Buck jumps back into the dating scene but fears he will return to his pre-Abby scoundrel-ous ways, while questioning if Maddie and Chimney are more than just friends. Emergencies involve a woman goes to great extremes on top of a freeway overpass to get the attention of her husband; A couple are involved in a car accident on their wedding day and a first date goes down the toilet.
Hen relives her evolution into the firefighter she is today as she recalls the first times she met Chimney and Athena.
The holidays are naughty and nice for the first responders, who must deal with a stampede at a toy store, a mistletoe stunt gone awry and neighbors competing in a Christmas-light showdown. Meanwhile, Bobby contemplates his future with Athena, Chimney and Buck attempt to lift Maddie's spirits and Eddie must decide if he wants to give his son what he really wants for Christmas - a visit with his mom.