A U.S. Army unit arrives in Iraq for their first tour of duty and quickly find themselves engaged in a bloody firefight. The squad emerges from the engagement essentially unscathed, though "Scream" and Lieutenant "Mad Cow" disagree over an interpretation of the squad's orders. A rash decision by Bo that jeopardized "Dim's" life gets him a serious chewing out from "Scream". When the soldiers go on a post-action "beer run", their truck contacts a buried improvised explosive device (IED) and Bo's right leg is blown off below the knee. "Dim's" wife, Vanessa, is shown to be cheating on him while he is away.

Bo tries to cope with his loss of a limb and a visit from his estranged, alcoholic father. After a bridge is destroyed, the unit is assigned the task of manning a military checkpoint. "Dim" clashes with "Smoke" over his racist views. During their first night on duty, the team is forced to shoot a motorist who approaches the checkpoint with lights off and ignores their orders to stop; a second car is allowed to pass. An Arab-American soldier from Detroit, named Tariq Nassiri, joins the squad. An explosive device located within the disabled vehicle is remotely detonated. On the second night a car again approaches with no lights and is fired upon; in this instance, the second vehicle to arrive is searched (after its occupants attempt to flee and are killed) and a young male insurgent is found hiding in the trunk.

The squad helps guard the insurgent prisoner, who is taken to a deserted town and undergoes both physical and mental stress at the hands of an intelligence officer to force him to reveal the location a shipment of stolen Stinger missiles. While in the town, the team must repel constant attacks by insurgents. In the hospital, Bo asks to have his morphine discontinued. Eddy finds himself alone when Vanessa fails to return home after spending the evening with her lover.

"Angel's" rifle skills prove invaluable when a supply convoy must travel through a hot zone. Vanessa's emotional state worsens when she suffers a traumatic loss. Terry makes the house wheelchair accessible in anticipation of Bo's return home.

"Smoke" is implicated in the death of a civilian, and an embedded reporter disappears after suggesting the death was an Iraqi setup. "Doublewide" sustains an eye injury when she is struck by flying debris, and Bo receives a prosthetic leg and begins the painful process of learning how to walk again. "Smoke" gets news that his mother has suffered a serious stroke. Vanessa withholds information regarding her miscarriage from "Dim." The journalist, who has quit his post, is kidnapped by insurgents.

The squad races to rescue journalist John Moffet before he is executed by his abductors, and in the process learns that a bounty has been placed on "Smoke's" life. "Mad Cow" is killed by an RPG round. Bo is discharged from the hospital and returns home. Vanessa is involved in a DUI traffic accident and leaves the scene; she is later confronted by MPs at her home who inform her that Eddy has been taken into protective custody in her absence. The squad makes video telephone calls to the States, wherein "Smoke" is able to speak with his mother and Vanessa tells "Dim" about the miscarriage and accident (though she lies about the circumstances regarding both). Moffet is beheaded and his captors are killed by gunfire just as the team attempts to effect his rescue.

Bo takes his frustration over his physical limitations out on Terry. Lieutenant Alexander Hunter is assigned as a replacement for "Mad Cow" and informs SSG Silas that he will do whatever it takes to get "Smoke" out of his unit; he also expresses his disdain for the group's "idiotic" nicknames (he is subsequently nicknamed "Underpants" by the squad). After two close brushes with death, "Mrs. B" is granted a short leave to tend to her son, who has been diagnosed with autism; instead, she visits the boy's father in prison. After the encounter, she makes a drastic decision regarding her future in the Army by going AWOL and boarding a bus to Los Angeles. When the fire team assists Iraqi guards at a makeshift prison, the situation turns critical when a prisoner taped with explosives threatens to blow himself and everyone else at the prison up. Ignoring the pleas from his fellow prisoners, the zealot detonates his makeshift bomb.

The squad is assigned to protect American workers constructing an oil pipeline in a remote village. The contractor offers to construct a building on behalf of the town when the Imam objects to the pipeline's construction. "Mrs. B" lands in Hollywood, falls in with an unsavory group, and is robbed. She is forced to steal food when she is unable to land a job, but after an act of kindness she seeks help returning home from a Marine Corps recruiter. Eddy is suspended from school due to behavioral issues, and Vanessa finally seeks out help with her drinking problem by joining Alcoholics Anonymous. After receiving a letter from the squad, Bo exerts himself and tries to run on his prosthetic leg. The unit's continuing presence in the Iraqi village proves to be a disruptive influence. When the pipeline contractor insists that the townspeople vote democratically as to whether a mosque or a school is to be built, the Imam's wife is declared to be an infidel; the group reacts violently, and begins to stone the woman, until the squad is forced to intervene. The contractor elects to route the pipeline away from the town, and the unit receives orders to move out, leaving the shamed woman to face an almost certain death.

"Mrs. B" returns to the unit and is instructed to remain silent regarding her actions back home. Vanessa confesses to "Dim" about her affair, who naturally reacts with disgust. The squad is ordered to sweep for insurgents and takes refuge in a palatial home they unattended. They quickly find themselves under fire, and "Dim" is pinned down. "Mrs. B" drives a humvee through the wall the attackers had been using for cover, killing one of them; the other is taken prisoner. "Scream" chews "Mrs. B" out for her rash actions, then tells her he will nominate her for a Bronze Star. The captive, hoping to secure his freedom, reveals a cache of gold and American currency worth five million dollars, which he tells them is now theirs. The unit ponders what to do with the money, finally deciding to take $10,000 for "Mrs. B"'s sick child. Once the booty has been secured for transport, the Lieutenant orders the unit members searched for contraband; a Cuban cigar found in the sergeant's possession is confiscated, though none of the money is turned up. That evening, "Dim" hands "Smoke" an envelope with instructions to mail it to the States for the care of "Mrs. B"'s son. "Dim" and "Mrs. B" seek comfort in each other's arms. Bo is unhappy when he learns that he is to be medically discharged.

Several of the squad members receive disturbing news from home. While "Angel" assists another unit in handing out food at a local village, a woman hurls an explosive device into the supply truck, killing herself and killing and wounding a number of Army personnel and civilians. Lieutenant Hunter and Corporal Shaver (a burned-out medic with a death wish) clash over proper triage procedures - the lieutenant wants Americans treated first, regardless of medical need. PVT Dane, who sustained a minor head injury in the blast, wanders off and is taken hostage by a distraught man whose son was also injured; the man threatens to kill the soldier with his own M4A1 if his son's wounds are not tended to immediately. "Angel" and the medic are sent to treat the boy, with orders to kill the man if need be to secure the release of the hostage. CPL Shaver is forced to amputate the boy's right leg due to blood poisoning, and although at first the procedure appears to be successful, the youth soon goes into convulsions and dies. When the man threatens PVT Dane, CPL Shaver asks to be the one who is shot because "I'm the one who killed your son." "Angel" attempts to wrestle the weapon away from the man just as the team bursts in, and the man is brought down in a hail of automatic gunfire. Much to everyone's shock, they realize that PVT Dane was also killed in the assault by an errant round, whereupon CPL Shaver jams the barrel of a pistol into his mouth and commits suicide. Back home, Bo struggles to adapt to his new physical condition, Sergio's relationship with another Army spouse heats up, and Vanessa tries to make amends for her drinking.

CPT Baron tries unsuccessfully to convince SSG Silas to re-enlist. The fire team is ordered to evict the residents of an orphanage that has been designated as being needed for "public use." Upon arriving at the site, the unit destroys a box they believe to contain a bomb, only to find out that it actually housed a young boy's chess set. The incident angers a young French woman who runs the orphanage. "Scream" gives the boy his own chess set as a replacement, and "Dim" accepts the boy's challenge to a game. Stateside, Terry discovers that Bo's father has been cashing his military paychecks. A suicide car bomb damages the orphanage building and kills the young chess player. "Scream" and the orphanage director begin to develop feelings for one another, and spend the night together. "Smoke" offers to play "Dim" in a game of chess, and (much to "Dim's" surprise) wins. "Scream" agrees to renew his enlistment under the condition that the Army makes proper provisions for the orphans. He informs his paramour of the arrangement, and tells her that it would be better for her, and the orphans, if the Army weren't around for a while (himself included). Sergio feels increasingly conflicted over his attraction to Anna. Bo drives to Abilene, over Terry's objections, to confront his father.

The fire team is burdened with the task of training a group of inexperienced Iraqi soldiers, then must put their lives in the men's hands as they search for an elusive, well-financed bomber. Bo confronts his father over the theft of his paychecks and ends up beating him into unconsciousness when the man attacks him. Vanessa experiences increasing difficulty staying "on the wagon." When "Doublewide" calls home to Sergio, she senses that something is wrong in his demeanor. Lieutenant Hunter arrives, seemingly on a mission to make things more difficult for the squad. After commandeering the Sergeant's tent, he makes it known that he wants to be intimately involved with all aspects of the search, the underlying message being that he wants the credit should the bomber be apprehended. He and "Scream" clash when the lieutenant fires into the darkness after seeing "someone running" (not taking into consideration that there are U.S. soldiers on sentry duty standing watch just outside the camp), and again over that best tact to take while questioning a detainee. One of the sharper Iraqi trainees makes an astute observation during the house-to-house search and convinces the team to return to the residence after nightfall. The lieutenant storms angrily onto the scene just as "Scream" gives the order to advance, and the bomber pops out of his hiding place and hurls a hand grenade at the group. Though no one is injured, the explosion gives the bomber a chance to flee; as "Angel," Tariq, the Lieutenant, and "Scream" pursue the suspect, "Scream" deliberately interferes with "Underpants" aim, allowing "Angel" and "Tariq" to take down the insurgent, after which "Scream" sarcastically congratulates the lieutenant for the kill. Sergio and Anna's plans to consummate their relationship are interrupted when Army messengers arrive with the news that Anna's husband, Enrique, has been killed in action.

Bo returns from Abilene and questions his ability to be a proper father, though Terry reassures him he is. "Dim" receives a video email from Vanessa apologizing for all of her past transgressions. After Lieutenant "Underpants" leads the fire team into an ambush, "Scream" asks Captain Baron (whose nickname, we find out, is "The Duke") to reassign the lieutenant. It is also revealed that the nickname "Underpants" comes from the lieutenant's apparent predilection for wearing red Calvin Klein briefs. "Angel" turns 21. Anna informs Sergio that she is moving back home with her mother, after which he leaves "Doublewide" a tear-filled voice mail message. Later, while leading a convoy of civilian vehicles, "Doublewide" is ordered to run down a young boy who is signaling the column to stop; when she steadfastly refuses, the lieutenant machine-guns the child out of fear of being ambushed. At the local track, Bo encounters another amputee who has overcome his disability and offers to help Bo stay in the Army. When Bo officially receives his Purple Heart, he is surprised to learn that he has also been awarded the Bronze Star (along with the "V" device) in recognition of his courageous actions. Back on the War front, when several convoy vehicles are hit by RPG fire, the lieutenant orders the squad to stop and fight off the insurgent attack. "Smoke" saves "Dim's" life after he was forced to engage one of the attackers in hand-to-hand combat, and Tariq is struck by an insurgent bullet from a Lee-Enfield but is saved from injury by his body armor. "Underpants'" reckless actions in trying to "take back" a panel truck results in the death of four soldiers. When "Dim" realizes that the Lieutenant's goal was to protect the "money truck" (containing 340 million Iraqi dinars), he berates the lieutenant for his callous attitude. Just as the lieutenant orders "Scream" to arrest "Dim" for insubordination, the truck is struck by another RPG round and explodes. When the smoke and dust clears, the lieutenant is lying face down in a pool of blood, shot through the back by "friendly fire." Afterward, as the squad members enjoy a beer around a camp fire and try to make sense of the day's events, "Scream" tells them to "...tell it to your shrink in a couple years. The lucky ones live to feel guilty."