The film begins in 2003 in a prison. A convicted murderer, Court Gentry, (Ryan Gosling) is visited by Donald Fitzroy (Billy Bob Thornton), a CIA recruiter. After going over Gentry's files and background, Fitzroy offers him a job working for the Sierra Program.18 years later in Bangkok, Gentry is now employed by the CIA as an assassin under the code-name "Sierra Six". He is tasked with taking out a target referred to as "Dining Cart" (Callan Mulvey). Aiding him is fellow assassin Dani Miranda (Ana De Armas), while superiors Denny Carmichael (Rege-Jean Page) and Suzanne Brewer (Jessica Henwick) keep eyes on him. Before Six can take out the target, he holds back when a child shows up. Six goes off the plan and goes after Dining Cart when the fireworks outside go off. He and Miranda take out Dining Cart's goons before Six goes after the man himself. After a fight, Six fatally stabs him, but Dining Cart reveals in his dying breaths that he is Sierra Four, and Carmichael sent Six to kill one of his own guys. Four gives Six a necklace with a chip containing information that can bring Carmichael and his cronies down.Carmichael contacts Six after, and he denies recovering anything off of Four. Six then switches clothes with a civilian before logging onto a computer to try and see what is on the chip, but he is lacking a code to log in. Six then contacts Fitzroy, who is in Baku, to inform him that he thinks Carmichael is a bad guy. Fitzroy says he will do what he can to look into it.Carmichael suspects Six knows the truth about him, so he employs the help of a former CIA agent, Lloyd Hansen (Chris Evans), to get to Six first before his deeds are exposed, even though Brewer is against it because Hansen is a known sociopath. Carmichael calls Hansen, who is in Monaco torturing a man with electric cables. Hansen is more than happy to go after Six. He finds Fitzroy at a funeral and questions him about Six, but gets no answers. Hansen resorts to having Fitzroy's niece Claire (Julia Butters) be kidnapped as leverage.Six is being escorted by an extraction team in Chiang Mai, when Fitzroy contacts the leader and tells him to quietly take Six out. However, Six is one step ahead of them and begins to fight and kill off the team before causing the plane to blow up and crash. He contacts Fitzroy, who tells him that Hansen has Claire.A flashback shows two years earlier in London, where Six and Fitzroy met with Sierra handler Margaret Cahill (Alfre Woodard). Fitzroy has to take on a mission after his address was leaked, so Six is tasked with babysitting Claire, who has a heart condition. Six bonds with Claire and takes a liking to her, even getting her medical attention when she faints. Some assassins break into Fitzroy's house at night, and Six kills them all before Claire gets out of bed to see what is going on.Back in the present, Six goes to Vienna to meet with his contact, Laszlo Sosa (Wagner Moura). While Six wants his help in getting him a new alias so he can go rescue Claire, Sosa betrays Six and drops him through a trap door so that he can collect the bounty on Six. Hansen and his goons arrive to get Six, but when Sosa demands his money, Hansen shoots him dead. Six sets up a trap as he fills the hole with water, setting off an explosion that takes Hansen's guys out. Six breaks out and kills the other goons before a brief fight with Hansen. Before Hansen can kill him, Six is saved by Miranda after she tranquilizes Hansen in the butt. Miranda is mad at Six as she is already being questioned by Carmichael after the Bangkok mission. She trans him too and puts him in a trunk.Hansen returns to Croatia where Claire is being kept in a mansion, and Fitzroy is brought back to her. Hansen also has his team there, along with Brewer, who chews him out for his actions. After his goons take Claire away, Hansen tortures Fitzroy by ripping off his fingernails for info on Six, but Fitzroy never budges.Six and Miranda go to Cahill's home in Prague now that she is retired. Six sent her the info on the chip, and she reveals to them that it contains all of Carmichael's shady deals and abuse of power. Hansen's guys manage to track them there and sends more assassins. After some gunfire, Cahill gives Six and Miranda a chance to escape. Once the assassins reach her, Cahill blows up her apartment with herself and the bad guys in there. Six and Miranda are chased through the city, leading to more gunfire, stabbings, and explosions as they kill off their pursuers. With all his teams dead, Hansen resorts to sending his own assassin, Lone Wolf (Dhanush).Six and Miranda go to a hospital where Six tracks Claire down using a signal from her pacemaker. Lone Wolf attacks them, leading to a 2-on-1 fight. Wolf tries to turn the police against Six before attacking the police himself. Six and Miranda manage to get away while Six tends to his wounds. There, he tells Miranda about how he killed his father for abusing him and his brother.The two make it to Croatia and initiate a full-on assault against Hansen and his goons. More assassins are taken out, and Six manages to save Fitzroy and Claire. As they make their way out, Fitzroy opts to stay behind to give Six and Claire more of a chance to escape. He uses a grenade to blow himself up along with more assassins, and Claire mourns her uncle. Meanwhile, Miranda fights Wolf, where they nearly kill each other until Wolf just gives her the chip, saying that he no longer wishes to work for dishonorable people like Hansen.Hansen catches up to Six and Claire, and he takes Claire hostage. Six chases him through a hedge maze before they end up at a fountain. Six drops his gun so Hansen will let Claire go. Despite her protests, Six tells her to run so she can be safe. The two men fight in the fountain, with Hansen almost gaining the upper hand until Six manages to overpower him. Before Six can kill Hansen himself, Hansen is shot dead by Brewer, who has had her own agenda all along. She also shoots Six and tells him they are going to pin the entire series of events on Hansen and paint him as a corrupt rogue agent. She also threatens Claire in order to keep Six in line.Carmichael and Brewer meet with their higher-ups and sweep the whole mess under the rug, with Carmichael destroying the chip to avoid any consequences. They attempt to make Six and Miranda be compliant in their cover up, but they are both shocked to find that Six has escaped their custody.Claire is now in a secret location with armed men guarding her. She hears gunshots and is met by Six, whom she hugs. He then takes her away so they can be free together.