Andromache “Andy” of Scythia, Booker, Joe, and Nicky are centuries-old warriors with regenerative healing abilities who use their vast experience to help people. Breaking their rule of working for the same people twice, they’re hired by former CIA operative Copley to rescue a group of kidnapped children in South Sudan. During the mission, however, they are ambushed and killed. After healing their wounds and killing their attackers, they realize that Copley set them up and filmed their regeneration.
Meanwhile, in Afghanistan, U.S. Marine Nile Freeman has her throat slit while taking down a military target, only to recover without a scratch. Upon waking, she shares a disturbing dream with the other immortals, who are then alerted to her existence. Andy tracks down and takes Nile before military personnel can move her to Germany, where she is due to undergo further testing.
Copley shows the video of the ambush to pharmaceutical executive Steven Merrick, who sends operatives to capture the team. Andy brings Nile to France, where Nile meets the rest of their team. She is told about the female immortal Quynh, the first of Andy’s comrades, who was cast into the sea in an iron maiden, and has been continually drowning ever since. The group also reveals that they are not truly immortal: their ability to heal eventually stops, without warning, which happened to a previous immortal, Lykon.
The group are ambushed by Merrick’s forces; Joe and Nicky are captured while a deceased Booker is left behind. After Booker regenerates, Andy kills their assailants but discovers that she has lost her immortality as her injuries do not heal. Booker locates Copley, while Nile separates from the group to reunite with her family.
Andy and Booker confront Copley, only for Booker to betray Andy by wounding her, arguing that Merrick might find a way to end the immortality they’ve both grown weary of. As they are captured, however, Booker realizes that Andy is not healing. Copley has a change of heart when he sees that Merrick is willing to torture the immortals indefinitely to study them. Nile, having realized Booker sold out the group, arrives too late to intervene, but she convinces Copley to assist her in a rescue mission. She storms Merrick’s London office and after she frees the rest of the immortals, they fight their way out through the rest of Merrick’s security, while Andy and Nile kills Merrick by pushing him out of the window.
As punishment for his betrayal, the group forbids Booker from contacting the rest of them for 100 years. The rest of the group meet with Copley, who reveals how their past missions had a greater effect than they ever knew, with the descendants of people they had rescued going on to help the world. With renewed faith in their mission, the group tasks Copley with keeping their existence a secret.
Six months later in Paris, a drunken Booker is astonished to meet Quynh in his apartment.