Higgins is writing his memoirs about a former Sandhurst classmate who now thinks he is Sherlock Holmes.
Patrick Macnee guest stars as Higgins' friend from long ago, who shows up at their friends funeral (in a flashback to a few years ago). The character portrayed by Patrick Macnee believes he is Sherlock Holmes, and that Higgins is the assistant, Dr. Watson. Higgins follows "Holmes" (being the faithful friend that he is) on the trail of Mr. Moriarty (so he believes).
Skip forward to the present where Higgins has locked himself in the library at Robin Masters' mansion writing the complete story of this haunting memory from the past.
Meanwhile, Magnum desperately needs to get into the library to retrieve the camera (needed for a case he is working on).
Magnum finally breaks into the library by picking the lock (after he tricked Higgins into taking "The Lads" for a walk) and began reading the manuscript about "Sherlock Holmes." At first he believed it was all fiction, but realizes it is a true tale.
- 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
- Gillian Dobb as Agatha Chumley
- Patrick Macnee as David Worth / Chee / Chinese Doctor
- Maurice Roëves as Hopkins (credited as Maurice Roeves)
- Michael Billington as Lever
- George Cheung as Victor Ching (credited as George Kee Cheung)
- Danny Kamekona as Hoon Ching
- Elizabeth Smith as Mrs. Akawi
- Galen Kam as Opium Pipe Salesman
- Bruce Atkinson as Vicar
- Higgins' involvement with the United Kingdom's secret intelligence service MI6 is mentioned again. Higgins also mentions his old friend Corky Bostwick again (for the fourth time).
- T.C. and Rick only appear in one scene of this episode.
- Magnum had a Cocker Spaniel named "Topper" when he was a kid.
- David Worth: I understand perfectly. More's the pity that you do not.
- Higgins: Will you please stop talking in bloody riddles and make sense?
- David Worth: Moriarty, man! Can't you hear plainly enough. Moriarty!
- Higgins: What on earth happened to you?
- David Worth: Judging from the small punctures in my side, some form of drugged dart. I eluded my captors and in short order, I found some materials for this disguise. An excellent mask, if I say it myself, eh? A Chinese ancient can learn more in a few minutes on these streets than an Englishman could ever discover.
- Higgins: David. I demand some kind of explanation.
- Higgins: David was returned to England and the safety of the sanitorium. He very kindly forgave me and we corresponded regularly, happily creating and solving the most complex crimes. Then several months ago I received a most curious letter from him, in part it read: "It's done Watson. I am now convinced my wits have dulled with age. I grant to Moriarty, his victory, but the good fight was waged, eh?" It was signed, most uncharacteristically, David Worth, Wellwood Sanitorium.
- Magnum: That's the last time you heard from him?
- Higgins: Forgive me for being such a bore that last few days. My obsession as you call it. It did seem, however, terribly important to finish it now.
- Magnum: He died?
- Higgins: Last week. Terribly sad for a man who so loved excitement, simple Pneumonia.
- (Magnum walks out)
- Higgins: Watson (smiles)