"Holmes Is Where the Heart Is" is the eighteenth episode of the fourth season of Magnum, P.I. which originally aired on March 8, 1984.
Plot
Higgins is writing his memoirs about a former Sandhurst classmate who now thinks he is Sherlock Holmes.
Synopsis
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.