Chicago police lieutenant Jim Brannigan is sent to London to extradite an American mobster named Ben Larkin. Upon arrival, he finds that Larkin has been kidnapped and it's up to him to navigate unfamiliar territory and work with Scotland Yard. Brannigan's brash American style clashes with the more reserved British approach led by Commander Swann, though they gradually develop a mutual understanding. Complications arise as Brannigan learns of an assassin hired to eliminate him, adding urgency to his mission.

As Brannigan delves deeper into the search for Larkin, he uncovers layers of criminal activity and corruption. He battles both local thugs and sophisticated criminals while keeping an eye out for the elusive assassin. Each encounter brings him closer to the truth but also puts him in greater danger. Persisting through numerous twists and intense action sequences, Brannigan's tenacity and street-smart tactics are put to the test as he pursues justice across the streets of London.