Marty thinks he's special and that's why he was chosen to be able to come back to earth. Professor Wyvern informs him that in fact a mistake had been made and a civil rights activist in South America named Martin Hopkirk, because of his exceptional life, was supposed to come back to earth, not Marty Hoplirk, the not so special P.I.
The original actors of the very much higher-rated Randall & Hopkirk (deceased) - Mike Pratt and Kenneth Cope - would often jokingly refer to their character's names as Marshall & Snelgrove, thus inspiring the names of the detectives in this episode.