In the 60's, the CIA agent Napoleon Solo is assigned to bring the mechanic Gaby Teller from East Berlin to the other side of the wall.
Gaby is the daughter of the scientist and American collaborator Udo Teller that defected from German at the end of WWII and now has vanished.
They are chased by KGB agent Illya Kuryakin but they succeed to escape.
Soon Napoleon Solo's chief Saunders discloses that Gaby's uncle Rudi works for the wealthy Alexander and his wife Victoria Vinciguerra and Udo might be secretly building a nuclear weapon for them.
Napoleon Solo is forced to team-up with Illya and Gaby and they go to Rome to investigate.