Adventurer-for-hire Victor McLaglen is hired by a political faction in a mythical European kingdom. McLaglen's job is to kidnap the young prince (Freddie Bartholomew) so that his employers can take over the government. The prince enjoys the experience because it releases him from the confines of protocol. When the prince is reluctantly rescued, McLaglen is thrown in prison. He escapes upon the realization that the political party he'd been working for actually plans to kill the prince and set up a dictatorship. McLaglen rescues the prince and preserves the Status Quo--and his blossoming friendship with the affable young monarch. Curiously enough, Professional Soldier is based on a story by Damon Runyon, taking a break from his "Guys and Dolls" chronicles.