In 2054, six months after enlisting for the United States Marine Corps, Privates Jack Mitchell and his best friend Will Irons are stationed in Seoul, South Korea, in an effort to repel a massive North Korean invasion force. The mission is successful, but at a massive cost: 6000 Marines are killed, including Will. Mitchell loses his left arm due to a piece of shrapnel severing it when he and Will were tasked to take down an aerial launcher, but is saved by his squad leader, Sergeant Cormack.
Whilst attending Will's funeral, Mitchell is approached by Will's father, Jonathan Irons. Irons offers Mitchell a place in his private military company, Atlas. Mitchell accepts and is given a "second chance" in the form of a high-tech prosthetic arm, as well as elite training and state-of-the-art gear. He befriends three fellow Atlas operatives: Ilona, Gideon and Joker.