Agatha Kimball, a wealthy blind woman, is being blackmailed and she hires Magnum to pay the blackmailer.
Having once been hired by wealthy, blind Agatha Kimball to find her missing grand-daughter Amy, Magnum's services are once again called upon by his elderly friend after she has been contacted by a black-mailer claiming to have some important information about Amy. She instructs Magnum to pay the man and get rid of whatever the information about Amy is without investigating further, but when Agatha is nearly killed in a series of suspicious accidents, and when the black-mailer is shot dead during the delivery of the ransom, Magnum decides he must ignore instructions and investigate just what trouble his elderly friend and her granddaughter are in. He delivers the money, only to see the blackmailer shot and killed by an unknown assailant after the delivery. Upon heading to Maui to learn the nature of the information that the blackmailer knew, Magnum discovers that the woman he knows as Amy is not actually Amy at all: she is a friend of the real Amy who wrote the letters to Agatha after the real Amy became addicted to drugs and then drowned in a tidal wave at the beach. The man who tried to save her but failed was the blackmailer, who turns out to have been hired by "Amy's" husband - the man behind the accidents. Magnum kills "Amy's" husband in a shootout, and he falls into the water. Once made aware of what has happened, Agatha willingly forgives Amy and proclaims that she is her granddaughter at heart.
- Tom Selleck as Thomas Sullivan Magnum IV
- John Hillerman as Jonathan Quayle Higgins III
- Roger E. Mosley as Theodore "T.C." Calvin
- Larry Manetti as Rick Wright
- Andrea Marcovicci as Amy Crane / Carol Foster
- Ted Danson as Stewart Crane
- Mercedes McCambridge as Agatha Kimball
- James Reynolds as Frank La Rue (credited as Jim Reynolds)
- Shirley Rogers as Denise
- Bobbi Dorsch as Girl
- Wallace W. Landford as Fat Jack (credited as Wally Landford)
- Kanani Choy as Eunice
- Lawrence Rodrigues as Caretaker
- Harry Chang as Doctor
- Shirley Rogers who plays Denise is the same actress who appears in the well-known scene in the opening credits where she is being taught to snorkel while Magnum holds her.
- Tom Selleck and guest star Ted Danson would work together again years later in the film "Three Men and a Baby" (1987) as well as its 1990 sequel.
- In the scene when Stewart threatens Magnum with a large hook and demands that Magnum give him the keys to the boat, there was supposed to be a fight between the two characters. Selleck objected to the logic of that, and instead, proposed another option: Magnum and Amy/Carol jump overboard.
- Mercedes McCambridge is well known for providing the voice of the demon in "The Exorcist." She also played Tom Selleck's mother in "The Sacketts."
- (Amy Crane is playing a variation on Alessandro Marcello's "Adagio in C minor" on the piano)
- Agatha Kimball: It's lovely.
- Amy Crane: You really think so? I thought you'd closed the door because of all the sour notes that I've been hitting.
- Agatha Kimball: Nooo, I was talking about your cologne scent, it's lovely. But your playing is lovely, too.
- Amy Crane: What if the words won't come out of my mouth?
- Magnum: They're not supposed to come out of your mouth. They're supposed to come out of your heart.