The movie begins in Los Angeles during a shootout between police officers and several bank robbers armed with automatic weapons. Bodies of policemen could be seen lying on the ground, as other officers gets shot. When the officers find that their normal means can't stop the attackers, they put out a request for assistance from the Special Weapons And Tactics (S.W.A.T) Unit. The group, which includes officers Jim Street (Colin Farrell) and Brian Gamble (Jeremy Renner) arrive on-scene, and prepare to infiltrate the bank. The police captain, Thomas Fuller (Larry Poindexter) attempts to negotiate with the suspects over a secure phone, attempting to get them to release several people being held hostage. One of the robbers has grabbed a woman and is holding her at gunpoint, using her as a shield. Gamble & Street infiltrate (disobeying orders) and Gamble shoots through the hostage's shoulder to end the standoff.
Back at police headquarters, Cap. Fuller is furious at Street & Gamble for their reckless actions, stating that the wounded hostage is now suing the city because of her injuries. Fuller removes both officers from SWAT as punishment, regulating them to "cage duty" (a degrading position involving the distribution of weapons & accessories to other officers). Gamble storms off in anger. When Street hesitates, Fuller offers him a chance to remain on SWAT if Street rats out Gamble for recklessness.
Street meets up with Gamble later on. Gamble believes Street has betrayed him, and is furious. Gamble attacks Street, and walks out-essentially leaving the police force behind.
Six months later, Street is exercising out on the beach one morning before work. He is still in the gun cage, performing menial tasks for other police officers.
A new police officer, Sgt. Daniel Hondo Harrelson (Samuel L. Jackson) comes to the precinct and asks Street to fix his customized rifle. Street notes the custom job and is impressed.
Hondo meets with his friend Lt. Velasquez (Reginald Cathey). Velasquez wants Hondo to form a new SWAT unit, a specially trained group that will hopefully counteract some of the bad publicity the department has gotten recently. Velasquez offers Hondo 2 of his own officers- T.J. McCabe (Josh Charles) and Michael Boxer (Brian Van Holt). Hondo agrees to them and decides to pick out 3 more candidates from the personnel files. Captain Fuller will have the final say on who goes into this new unit.
Street comes home that night to find that his girlfriend is moving out, feeling that their relationship has run its course. Next day, Street goes back to the cage and shows off a new idea he had for the SWAT teams: a gigantic hook & chain designed to bring down barricades and end hostile standoffs. Sgt. Hondo comes in to collect his weapon and is impressed by Streets' work.
Out at the officers' targeting range, Hondo and several others are practicing target firing. T.J. manages to out-shoot Hondo, but Hondo bets $200 that Street is a better shot. T.J., not feeling threatened by a "gun bunny" like Street, agrees to the challenge. They face off, and Street ends up victorious.
Meanwhile, a young man named Alex (Oliver Martinez) arrives at Los Angeles International Airport and heads out into the city. He intends to visit his uncle's favorite restaurant as a birthday surprise.
Next day in the Gun Cage, Hondo recruits Street into being his driver for a day in order to recruit more SWAT members. They first investigate an officer named Deacon Kay (LL Cool J), who is found in the middle of a foot pursuit with a suspect. Hondo intervenes and ends the pursuit early, angering Deke. But when Deke finds out that Hondo is an agent from SWAT, his attitude changes. Hondo invites Deke to study at "SWAT school."
After deciding against another candidate, Hondo & Street go to speak to a police officer named Chris Sanchez, who has applied and been rejected to SWAT three times previously. The answer to this is soon obvious- Officer Sanchez is a woman (Michelle Rodriguez), and women are not usually permitted to join the SWAT teams. Despite this, Hondo offers her a spot on the team.
Upon returning to headquarters, Hondo asks Street if he wants to fill the last spot on the team. Street is hesitant, knowing that Captain Fuller is very unlikely to approve him for SWAT. Street also confirms that he did not say anything bad against his old partner, Gamble.
Alex comes to a local gourmet restaurant and meets his uncle, who is shocked at his nephew's arrival.
When Hondo meets with the Captain, Fuller rejects both Sanchez & Street, but Hondo refuses to back down. Fuller makes a compromise- he will accept the group for now, but if they fail the preliminary trials needed for SWAT, then Street & Hondo will both be taken off the force. Hondo agrees, confident in his group.
Alex speaks with his uncle about the family business. It appears that Alex's uncle has been stealing money from the family for some time, and Alex has come to exact revenge. Alex stabs his uncle and leaves the restaurant, but is soon pulled over by traffic cops for a broken taillight. Since the car Alex is driving is his uncles' (and there is an arrest warrant out for that name), the officers detain him for questioning and verification of his identity.
The next day is the first day of SWAT school. The new squad members get to know each other, and it's revealed that Boxer's sister was formerly involved with Street- so there is some friction between them. Hondo puts the group through several tests- including infiltration, armor, weapons, physical conditioning and long-range shooting. Despite some initial difficulties, they are soon performing admirably.
Before long, the time has come for the team's final evaluation. Fuller has set up a unique scenario- the team will have to infiltrate an airplane taken over by "terrorists" (Played by fellow officers armed with paintball guns) ; they will have to take back the aircraft and take out all six hijackers within the time limit. While looking over the plane's schematics, Street realizes that there is an alternative entry point for the plane: a tiny elevator shaft which only one member of the team could fit through. Sanchez jumps at the chance. They move into the plane, and T. J. is "killed" by a paint-ball in the process. Despite this, the group takes out all the hijackers and sets a new record for the course in the process.
That night, the group celebrates by eating at a cafe owned by Deke's father. Hondo congratulates them, but also admits that he is not too happy that T. J. wouldn't have survived if they had been in a real operation.
Alex is still in jail, frustrated because his fingerprints do not match his identification and thus he must remain locked up.
After a few hours of eating and drinking, most of the group heads home. Sanchez follows Street to his favorite bar and witnesses a confrontation between Street and his ex-partner, Gamble. Sanchez then invites Street to her daughter's birthday party taking place the next day.
Since the next day is Sunday, nearly all members of the new SWAT team are relaxing. T.J. is on a date, Hondo is on the golf course, Deke is at the store with his wife & sons, Boxer is at home with his wife watching TV (Ironically watching the 1970's program that inspired this movie). Street has taken Sanchez up on her offer and is now entertaining the kids at Eliza's birthday party. Suddenly, everyone's pages are activated- the SWAT team is being called to action!
Lt. Velasquez outlines the situation- a crazy man has barricaded himself in a house and is taking shots at officers through the windows. The man also claims to have wired all the entrances with explosives. Street volunteers his grapple weapon, and the team uses it to tear down the man wall of the house, ending the standoff in seconds.
At the sheriff's department, a deputy finds some details pertaining to Alex's true identity. Velasquez is notified and asks Hondo's unit to intercept and escort the prison bus carrying Alex. At the same time, Alex's accomplicies ambush the prison bus, killing two deputies and almost letting Alex get away before the SWAT unit arrives.
Later, Captain Fuller gives a press conference confirming that the man they have in custody is Alex Montel, an international fugitive wanted in over a dozen countries on charges including murder, arms dealing & drug trafficking. As Montel is being led to a holding cell, he faces a TV camera and pledges $100 million as a reward for anyone who breaks him out of jail.
Hondo's unit is preparing to evacuate Montel via helicopter when the chopper is shot down by an unseen assailent. They take Montel back inside and form a new plan- to lead the fugitive away via a motor parade. Several gangsters attack the vehicles, hoping to free Montel. But the vehicles turn out to be a decoy, and Alex is being led down a private access area into some unmarked SUV's. One officer is killed during the attack on the decoy and two more are badly injured.
As they proceed through the city, Boxer mentions to Street that his sister is not too happy with her current situation, and thinks that Street should call her- thus mending the rift between those two.
Halfway through the second checkpoint, T.J. stops the car. He has decided to take Alex's offer. As they attempt to reason with him, Boxer is shot by T.J.'s parter- Gamble. Street screams out for help via the radio, and the rest of the team shows up within seconds. As paramedics arrive to treat Boxer, Hondo & Street chase Gamble's group into the local subway station. They miss the train and hope to catch it at the next station. But Deke & Sanchez report that the train did not arrive at the station, so they give chase in the tunnels.
After nearly falling into an explosive trap set by Gamble, the team arrives outside the tunnel. At this point, Captain Fuller is furious- he suspects the entire team might be in on T.J.'s betrayal. He orders Hondo and his crew to stand down.
As they drive out, Sanchez notices a low-flying jet over the city. They decide that the jet must be Gamble's plan for evacuating Montel. Fuller has previously sent all possible reinforcements to the local airport in an attempt to cut off the potential escape. Hondo & his group follow the jet, which is actually landing on a bridge rather than at the airport. Hondo gets radio confirmation that Gamble has hostages on board the airplane, and they need to stop the craft immediately to save innocent lives.
Hondo drives their car into the landing gear of the plane, crippling it. A gun battle erupts between SWAT and Gamble's team. Sanchez is hit in the shootout, receiving a minor shoulder wound. Alex makes a break for it, but Deke follows (echoing his earlier foot pursuit) and re-captures Montel.
Hondo confronts T.J. inside the plane. T.J. asks about Boxer. Hondo is hesitant, but confirms that Boxer has received treatment and will presumably survive from his injuries. T.J. pulls out his gun and shoots himself in the head, preferring death over facing his actions.
Street chases Gamble into a train yard, where they have one last fight. Street prevails, and Gamble is decapitated by a passing train.
Fuller arrives, commending the team for their work, but pointing out that the job is not yet finished. They take a SWAT bus out to the nearest Federal prison, finally releasing custody of Montel to the proper authorities.
As they sit on the bus, heading home, a report of a robbery in progress comes onto the police band. Hondo points out that their shift has technically been over for 12 hours. Despite this, the team agrees to mount up and head out to their next mission.