At a Chicago city firehouse in 1971, a young boy and his even younger brother are trying on a fireman's coat. The older boy, Steven McCaffrey, lectures his brother, Brian, on how to properly buckle the front so it won't open in a hazardous situation. Just then, the alarm goes off in the firehouse and their father rushes in, asking Brian if he wants to join them on the call. Brian does and his father takes him in the fire engine with him. He shows his son how to properly blow the engine's horn to clear the streets and intersections on their way to the site.
When they arrive on site, the call is for an apartment building. Brian watches as his father quickly makes his way up to the top floor where the blaze is. He and his partner, John "Axe" Adcox, rescue a kid about Brian's age and give him to their cohorts waiting on a nearby ladder. McCaffrey and Axe continuing battling the fire when a gas line ruptures and the entire floor explodes, killing McCaffrey. Brian watches his father's helmet land in front of him on the street. Axe, pushed away from the explosion by McCaffrey, rushes out and hugs Brian, then appears to bark orders to the rest of the company. Brian is stunned and picks up his father's helmet just as a photographer snaps a picture of him. (The picture later becomes a prize-winning cover of Life magazine.)
Twenty years later, in the present day, Brian is at a party being thrown at a local bar; he has just graduated from a firefighter's training academy and the class is celebrating. He and his friend, Tim, are also waiting for their station assignments. Tim is given Engine 17, the toughest company in the city. Brian's own assignment is elsewhere because he'd bribed the city fire chief. Brian doesn't want to work for 17 because his older brother, Steven, runs the house and the two have become estranged. At the party, Brian also meets up with his old girlfriend, Jennifer, who isn't very happy to see him; Brian has hardly made any effort to keep in touch with her or his family for several years. Tim interrupts them, saying that 17 has been sent on a nearby call and that they both should go and watch their comrades work.
Moments before, after parking his Porsche in front of his house, a man named Seagrave is entering his house when he's consumed by a huge explosion that propels him back into the street and through the windshield of his own car. When Brian and time make it to the site, the fire is already out and Brian's brother and Axe are there, bragging about the completed job. Brian sees Seagrave, horribly burned and dead, still embedded in his windshield. Steven appears and tells Brian that he's talked to the fire chief and convinced him that Brian belongs at Company 17. Brian is flustered that his brother interfered with his station assignment. Meanwhile, Donald "Shadow" Rimgale, an arson inspector, arrives on site and begins to collect forensic evidence. When Steven asks him if he's found a cause, Rimgale smartly answers "Mice, with matches."
Brian visits his sister-in-law, Helen, and his nephew. He also notices that Steven isn't home; Helen tells him they've been separated. Because he didn't keep in touch with anyone back home, Brian didn't know about their situation. He next goes to his father's fishing boat near the river, where Steven has been living since the separation. Steven is suspicious of Brian's return to firefighting and suggests that Brian won't stay in the job, much like his failure at several other career fields of the previous few years. Brian tells his brother he's wrong.
The next day, Brian gets up early for work, however, his car won't start and he must run to 17's house. When he gets there, the company is already on their way out to a call. He jumps aboard Steven's engine and dresses inside, catching up with Axe, who had looked after the McCaffrey brothers after their father died. The company reaches the site, a dressmaker's factory. After Steven embarrassingly buckles Brian's fire coat for him, they go inside and begin to battle the fire, which turns out to be quite difficult; it seems to be "jumping floors". At one point, the floor collapses and the men have to save one of their buddies and pull him up safely. The team receives a call saying there are no other companies to help them out. Brian is helping Steven with his hose when he thinks he hears someone calling for help. He rushes into another room but is blown backward by a blast of fire. When the fire backdrafts, Brian sees someone lying unconscious nearby, whom he carries outside, only to find that it's a mannequin, not a real person. Later, while he vomits outside, Steven yells at Brian for wandering off. Back at the fire station, Brian and Tim, as new candidates (or "probies") are ordered to make lunch for the entire company. The company welcomes them both and plays a prank on Brian, using the mannequin he'd "rescued" from the building.
At a party being thrown for the Chicago Fire Chief, Steven spots his wife with another man. A Chicago alderman, Swayzak, offers Brian a chance to work with Rimgale. Brian turns him down and finds out that his old girlfriend, Jennifer, works in the alderman's office. Steven grows increasingly jealous of his wife and also becomes quite drunk and starts a fight with the man. Brian tries to help and they're both ejected from the party. Brian takes Steven home and Steven, before passing out, tells Brian that fire itself will not kill him.
Later, Brian and Steven respond to a fire raging in a slum tenement where they're told they'll find a trapped kid. The two rush into the building without waiting for backup. When they find the apartment where the child is trapped, Steven breaks the door down and rushes in. Brian hesitates and is blown back by an explosion of fire and air. Backup arrives and Steven saves the young boy. Later, the two talk on the street and argue about Steven's seemingly uncontrollable need to emulate their father which often gets him in trouble and risks the lives of his crew. Brian decides to leave Company 17, take Swayzak's offer and goes to work for Rimgale. Rimgale takes Brian out with him immediately to the parole hearing of a pyromaniac, Ronald. When Brian first meets Ronald, the man is excited, recognizing Brian from the cover photo of him from Life magazine. He also shares the story of Rimgale's nickname, "Shadow"; Rimgale had been burned badly in a fire started by Ronald years before and the blast from a small tub of phosphorus had thrown Rimgale's shadow on the wall (Rimgale still carries horrific scars on his back and side from the incident). At the parole hearing, Ronald says that his incarceration has reformed him and he's ready to rejoin society. Rimgale isn't convinced and interrupts the hearing to show Ronald a burned doll from one of his child victims. Ronald admits he'd like to burn the entire world and his parole is denied.
At a city theatre, the proprietor, Donald Cosgrove, opens his office door and is killed by an escaping blast of heat. Company 17 responds, but most of the blast, a "backdraft" has burned out. Brian and Rimgale arrive to investigate the scene. Brian's former colleagues from 17 seem somewhat resentful that he's taken a desk job. Rimgale searches the proprietor's office and finds a clue behind an electrical socket in the wall. When they talk later with the county coroner, Brian and Rimgale are told that the accelerant from the fire was a chemical called trichtochlorate, traces of which were found on Cosgrove's corpse.
Brian reunites with Jennifer and shows her around Rimgale's firehouse. The two climb onto the back of an engine and have sex, just as the company is called to a high-rise fire. Company 17 is also there. While inspecting the burning floors, Steven takes the opportunity to train Tim on breaking doors down with their axes. Tim forgets to check one of the doors for heat and is blown back by a blast of fire when he breaks it open. Another blast sets him on fire. By the time the rest of the company comes in with hoses, Tim is horrifically burned and taken away in an ambulance. At the hospital, Brian and Steven get into a fight when Brian suggests his brother's need for heroics caused Tim's injuries.
Brian asks Jennifer to do some snooping in Swayzak's office for any evidence that could link the deaths of Seagrave & Cosgrove. Rimgale is convinced they were all victims of the same arsonist. Jennifer finds evidence that they were friends of Swayzak and were involved in a complicated fraud scam that forced the closure of firehouses throughout the city; the firehouses would later be turned into parks or community centers, costing firefighters their jobs and resources. Rimgale and Brian go to Swayzak's home. When they smell escaping natural gas, they search the house. Brian finds Swayzak unconscious and is attacked by the arsonist, who wears a ski cap. Brian wrestles with him and shoves him against a shorting electrical plug (the one meant to set the natural gas ablaze). Rimgale finds them and throws the man off Brian. The arsonist runs off. Brian and Rimgale pull Swayzak from the burning house just as it explodes. Rimgale lands on a wrought-iron railing and is impaled through his shoulder. While angrily recovering in the hospital, he charges Brian with continuing the investigation.
Brian visits Ronald in prison to try and understand the psychology of the arsonist. Ronald gets Brian to admit that he'd always wanted to be a firefighter because of his father and his untimely death. Brian also admits that fire itself still scares him, much like it did the day he saw his father burn to death. Ronald suggests to Brian that the arsonist is likely a firefighter who has access to trichtochlorate. Brian suspects his brother and goes to his father's boat, finding cans of a solvent containing the chemical. Steven shows up and the two argue over Steven's responsibility in raising Brian and Brian's failure to be a firefighter. Steven counters, saying that he found the responsibility of taking care of Brian after the father died thrust unfairly upon him years before. Brian goes to 17's house and searches Steven's locker, finding nothing. As he closes it up, he sees Adcox, shirtless, in the locker room. He notices a strange burn on Adcox' back, shaped like an electrical socket. Brian realizes it was Axe that he fought in Swayzak's house and he is the arsonist.
Outside, in an alley, Steven asks Brian if the arsonist is Adcox, not noticing Axe listening at the window above them. The company is called to a fire at a chemical factory and Brian joins them. A few blocks from the site, the truck Brian is riding in flips over in a minor auto accident and Brian must run to the factory. He finds Steven and Axe arguing on the roof; Axe explains that he was fed up with politicians closing down firehalls, reducing the number of firefighters in the city and placing the lives of the remaining fighters in danger due to downsizing. Axe's plan was to kill all the politicians responsible. While the three continue to debate Axe's actions, the roof of the building begins to collapse. All three run to the edges of the roof and Brian makes it to a fire escape. A blast of fire causes him to fall into a large elevator shaft that fills with water. A broken gas pipe throws fire in his direction but he is saved by Steven. As they walk out of the building, they are attacked by Axe. Axe and Steven seem ready to fight each other with their axes but Adcox breaks down. A blast from a few chemical barrels below them wrecks the catwalk they're standing on and they end up hanging from the twisted metal, with Steven holding on to Axe several stories above the inferno below. With Steven's hand slipping, Axe tells Steven to let him fall. Steven refuses and his hand slips. Axe is killed and Steven falls onto a pipe-railing, causing serious injury to his abdomen. Two of 17's other men rush into the room but lose their hose in another blast. Brian makes it down to the floor of the massive room and takes control of the hose, allowing the two men to reach Steven. Steven is proud that his brother has overcome his fear of fire and is bravely fighting the blaze.
Steven is rushed to the hospital, with Brian at his side. On the way, Steven's condition worsens and he falls unconscious and dies. Before he dies, he implores Brian not to reveal that Axe was the arsonist so it won't taint the department. Following a traditional funeral procession through the city, Steven and Axe are buried with honors. Brian and Rimgale interrupt a press conference being held by Swayzak and present the evidence they've collected, stating that Swayzak engineered the downsizing of the Chicago fire department along with Cosgrove, Seagrave and others. Brian apologizes to Jennifer for bringing down her boss.
Brian rejoins 17 and keeps the shield from his brother's helmet. A call comes up and Brian joins the company on the run. On the way, he helps a new candidate buckle his fire coat properly, much like his brother had done for him.