It's the year 2020 and Charlie Kenton (HUGH JACKMAN) is a former boxer who was put out of work when huge, remote controlled robots were introduced to replace humans and provide audiences with an even more violent version of the sport. Now, he tries to make a living pitting old robots against the likes of bulls as publicity stunts to make money.
But he's had no luck with that, resulting in gambling debts owed to the likes of bookie Ricky (KEVIN DURAND) who wants his cash. He also owes money to his former girlfriend, Bailey Tallet (EVANGELINE LILLY), who runs the gym where Charlie once trained under her father.
To make matters worse, Charlie now learns that his 11-year-old son, Max (DAKOTA GOYO), is back in his life. Charlie wanted no part of fatherhood and thus signed away his custody rights to the boy's mother, but as she's now passed away, Max is back. Fortunately for the boy, his Aunt Debra (HOPE DAVIS) wants custody, something Charlie is all too eager to sign over, especially when he realizes that Debra's husband, Marvin (JAMES REBHORN), is loaded and will essentially "buy" the boy for his wife.
As they're going away for the summer, however, the deal is that Charlie will watch over the boy for the next few months, when he really wants to pass off that responsibility to Bailey. But Max is intrigued by the world of robot boxing, and thus tags along, seeing his dad make bets with other fight bookies such as Finn (ANTHONY MACKIE).
When his latest 'bot loses badly, Charlie thinks he's done. But things look up when Max accidentally unearths an old sparring robot that he decides to fix up as his own. It turns out Atom can take quite a beating and, as controlled by Max or Charlie, can deliver the same, all of which results in a winning streak. As they head toward a potential showdown with the robot boxing champion, Zeus -- created by Tak Mashido (KARL YUNE) and promoted by Farra Lemkova (OLGA FONDA) -- the father and son find their relationship changing into something unexpected.