The opening text states that there were over 200 American outposts in Libya until 2012. Following the 2011 civil war, the deposition and execution of dictator Muammar Gaddafi, Benghazi became one of the most violent places in the world, forcing the United States to pull their outposts out of there, all except for one called The Annex, which is protected by a team of CIA contractors, the Global Response Staff (G.R.S.).
Jack Silva (John Krasinski) arrives in Benghazi and is picked up by his best friend/fellow contractor Tyrone "Rone" Woods (James Badge Dale). The two are stopped in the streets by a group of militants. Rone contacts their Chief (David Costabile) to bring in the rest of the team for help, but the Chief refuses. The militant leader orders the two to pull over their vehicle for inspection. Jack and Rone pull out their guns for defense, and Rone tells the leader that there is a drone circling them and will get him and his family if they do anything to him and Jack. The two are allowed to leave.
At the CIA safehouse called The Annex, Rone confronts the Chief over not letting the team go in. The Chief reminds Rone not to engage civilians and avoid conflict with the militants. Jack meets the rest of the team - Kris "Tanto" Paronto (Pablo Schreiber), Dave "Boon" Benton (David Denman), John "Tig" Tiegen (Dominic Fumusa), and Mark "Oz" Geist (Max Martini).
Jack goes on a brief task with two undercover CIA officers - Sona Jillani (Alexia Barlier) and Brit Vayner (Freddie Stroma). The rest of G.R.S. is nearby for protection. Rone spots a couple of suspicious men and gets out of the car, compromising his position. Jack is alerted and gets Sona and Brit out of there.
The team is brought to the Special Mission compound where the U.S. Ambassador to Libya, J. Christopher Stevens (Matt Letscher) is staying. They meet other guards, Dave Ubben (Demetrius Gross) and Scott Wickland (David Giuntoli), and become familiar with the location.
On the morning of September 11, 2012, Stevens notices some dubious men around the compound taking pictures, prompting him to notify his security detail. Meanwhile at The Annex, the team contacts their families. Jack talks to his wife Becky (Wrenn Schmidt) and their daughters. One of the girls spills the beans to Jack that Becky is pregnant with their third child.
That night, a large team of militants storm the compound and attack as the Libyan guards on the outside surrender. The militants fire off shots and invade the compound as Wickland takes Stevens and Sean Smith (Christopher Dungli) into the panic room. Knowing they can't break in there, the militants begin pouring diesel fuel all over the floor to burn the men out. The fire is lit, bringing heavy smoke into the panic room. Wickland makes it out but ends up losing Stevens and Smith. The men at The Annex are informed of the attack and want to head out to help, but the Chief refuses to send them out for fear that The Annex will be defenseless. The team disregards their orders and heads over to Special Mission. After arriving, Jack and Rone find Smith dead from smoke inhalation. The team heads back to The Annex but are pursued by the militants with RPG's and AK's. The men fire back against the militants.
Fearing retaliation, the CIA officers attempt to contact the nearest airbases to aid in an strike against the militants. They are able to contact Glen "Bub" Doherty (Toby Stephens), a G.R.S. officer in Tripoli that rounds up his own team and heads down to Benghazi to help.
More militants head toward The Annex and continue their attack against the Americans. The team fires back and continues to defend the compound, despite coming under heavy fire. The militants fall back and try to regroup, as does the team. Glen and his team arrive to help. The team is contacted to learn that Stevens' body was found, and that he died of smoke inhalation.
The Annex is contacted and told that more help is on the way. Soon, the militants return and launch a mortar strike against The Annex. Rone gets hit and is killed, as is Glen moments later. Oz is wounded and has his left arm mangled. Jack finds Rone's body and weeps for his best friend.
As dawn arrives, the team is left vulnerable. A large convoy of vehicles head toward The Annex. Tanto has his gun aimed at a couple of men from the vehicles, but he gets a signal from them that they are there to help the team. Tanto cries with relief. These men from the Libyan army gather the Americans and bring them to an airfield. The bodies of Stevens, Smith, Rone, and Glen are recovered and taken. The Chief chooses not to get on the plane back home until Jack confronts him and orders him to after reminding him he is partially responsible for some of the lives lost in the attack.
Before departing, Jack calls Becky to let her know he is okay. He breaks down crying when he tells her that Rone didn't make it. As the plane arrives, Jack uncovers Rone's body to look at him one last time.
The ending text states that 28 Americans were rescued that day. Libya was declared a failed state and is now a stronghold for ISIS. Ten days after the attack, thousands of Libyan civilians gathered to mourn Ambassador Stevens. The Chief and surviving members of G.R.S. received medals in a private ceremony, and the men subsequently retired to be with their families.
The final shot is of two stars on the wall of the CIA HQ for Rone and Glen.