John McClane travels to Russia to help out his seemingly wayward son, Jack, only to discover that Jack is a CIA operative working to prevent a nuclear-weapons heist, causing the father and son to team up against underworld forces.

The story begins when a scientist is working on an intelligent robot for humanitarian purposes, who discovers the company he works for is intends to use the robots for military purposes. The scientist steals one of the robots which ends up being lost and broken, then is fixed by a lonely 11-year-old boy. A robot boy and a human boy team up to save their respective parents, who are being held captive by the organization that funded the robot's creator. The two become friends and embark on adventures while being pursued by the authorities.