He travels back to Jeff's apartment to wait for him, whereupon Jeff turns up in the early hours of the morning with a beautiful girl in tow. Marty demands Jeff send his date home and call the police, which he eventually begrudgingly does, travelling with Inspector Clayton to the house where the murder took place. However, when they arrive they find his wife and it appears that Mr. Howarth is back alive and well.
Jeff leaves after Howarth threatens to sue him, only to arrive back the next morning to apologise to Mrs. Howarth, again at Marty's insistence. Marty sees Rawlins in the study and calls Jeff in, by which time Rawlins has disappeared into the secret room branching off it.
Engrossed with the case, Marty spends time in the Howarths' house, whereby he learns that both Mr and Mrs Howarth have been murdered and replaced by impostors. Insisting Jeff returns after he locates the bodies in the basement, Jeff returns to the house again, only to find the bodies are no longer there.
Jeff - now in serious trouble - uses Jeannie as an alibi to escape a third police warning, but is growing increasingly concerned by Marty's behaviour. He visits a psychic medium Professor Plevitt at the British Museum, who convinces Jeff that Marty like all ghosts is hallucinating. Driving home, as a result of Plevitt's advice Randall disbelieves Marty's warning that Rawlins is out to kill him, only convinced when he narrowly avoids being shot in his car.
Later Rawlins turns up at Jeff's office pretending to be a James Wenworth-Smith of Westminster". However, Marty warns Jeff and the two struggle, whereupon Jeff is knocked unconscious by the highly physically trained Rawlins and tied up back at the Howarths', with their plan to frame him for the murders.
However, under Jeff's guidance Marty visits Professor Plevitt in order to get him to call the police. The process takes a great deal of time and Marty finds it extremely difficult to convince the medium that he is an extraordinary ghost with excellent capabilities. It appears that disguises were being used to impersonate the murdered Howarths (the female agent's name is revealed as "Karen"), and that Mr. Howarth was in charge of savings for the Secret Service. His murder meant that the enemy impersonators had access to the full names and details of every undercover agent in the UK. The police arrive and arrest all three, though Marty winds up Jeff by pretending to hallucinate for seeing a body in a cupboard, getting his revenge on Jeff for his disbelief.
Although the 4th episode in the series, Never Trust a Ghost was the 14th episode to be shot, filmed between December 1968 and February 1969.
A small amount of location shooting was done for this episode: the exterior of Jeff's apartment on St Johns Wood High Street, where Jeff is seen arriving in act one, Jeannie arriving by mini and the arrival of the police car with Inspector Clayton in act two. When Jeff drives back from his visit with Dr Plevitt, exterior footage of the Vauxhall was shot on Great Chapel Street and Oxford Street. Second-unit footage of the exterior and street of Adams Furniture in Harrow for the sequence showing Jeff driving back to the Randall & Hopkirk office in the Vauxhall and being shot at by Rawlins, who then drives off.
Back projection footage seen behind Jeff and Marty in the studio shot car interiors consisted of footage of Goodge Street, Charlotte Street, Tottenham Court Road and Howland Street. Stock footage depicting London at night at the start of the episode consisted of shots of Berkely Street, Lambeth Pier and the Houses of Parliament. Stock shots of New Scotland Yard and the British Museum were also used. The street and exterior of Howarth's home were shot on the standing street sets on the backlot at Elstree studios.