To Kill the King (1974)

Lance Henriksen is a successful restauranteur who is having a hot n' heavy affair with the wife (Susan Tyrell) of a Presidential Adviser. During one rendezvous they overhear the husband and an Admiral discussing "the killing of the patient", and as the adulterous couple have been under surveillance this information reaches a top Federal Agent (Patrick O'Neal) who provides security for the President. His investigation begins to cast light on a conspiracy within government to assassinate the President - and now he must expose the conspiracy and stop it cold.