My Late Lamented Friend and Partner

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 dies in the bathtub, supposedly of a heart attack, but Marty thinks otherwise. Before he can convey his feelings about the woman's death to Jeff, he dies. When Marty's death is dismissed as a hit-and-run, his ghost returns, visible only to Jeff, to tell him it was murder.