A Force of One is a 1979 martial arts film starring Chuck Norris, Jennifer O'Neill, Ron O'Neal, Clu Gulager and Bill Wallace. The film was directed by Paul Aaron and written by Pat E. Johnson and Ernest Tidyman.
When a team of undercover narcotics officers is targeted by a serial killer, the police recruit karate champion Matt Logan (Chuck Norris) to bring the murders to an end. Narcotics Officer Amanda "Mandy" Rust (Jennifer O'Neill) discovers that a traitor within the police ranks is behind the killings.
(Summary from Wikipedia)