Randall and Hopkirk (Deceased) (1969) - 1x01 - My Late Lamented Friend and Partner- Followmy.tv

When private investigator Jeff Randall goes out of town after collecting evidence for a woman about her cheating husband, partner Marty Hopkirk takes his place to settle the account. While waiting to see the woman, she is killed in her bathtub by an assassin. Although her heart condition is blamed, Marty starts asking questions. Before he can tell Jeff his suspicions, Marty is killed intentionally by a hit-and-run driver. However, he returns as a ghost, visible only to Jeff, and together they search for the killers.