When private investigator Jeff Randall goes out of town after collecting evidence for a woman about her cheating husband, partner Marty Hopkirk takes his place to settle the account. While waiting to see the woman, she is killed in her bathtub by an assassin. Although her heart condition is blamed, Marty starts asking questions. Before he can tell Jeff his suspicions, Marty is killed intentionally by a hit-and-run driver. However, he returns as a ghost, visible only to Jeff, and together they search for the killers.
Jean and sister Jenny arrive at the office to find Jeff in the middle of a knock-down, drag-out argument with Marty over the use of Jean's car (which Marty, although dead, still considers his). The combination of lack of work, impending bills and Jeff's car stolen and used in a robbery that the police suspect him in lead Jean and Jenny to think that Jeff has snapped. They plant a tape recorder in the office to gather evidence, which they present to a psychiatrist who is behind a crime spree. The psychiatrist must reprogram Jeff to forget about Marty in order to prevent Marty from relaying information about the crime spree.
When things start flying around her apartment, Jean comes to the conclusion that her late husband Marty is trying to contact her. She has fallen for the trap set by the Foster brothers, who claim to be ""scientific spiritualists"" and approach Jean with a job offer: she will have introduce them to people who are interested in contacting ""the other side"". What she doesn't know is their ""scientific approach"" involves murder.
A government employee, James Howarth, is murdered in his home. ""There were no witnesses"", the murderer boasts. But there WAS a witness: Marty Hopkirk. Trouble arises when Marty convinces Jeff to investigate. He finds the ""murdered"" man very much alive -- and upset. Marty stays on the case, but every time he convinces Jeff that something is wrong Jeff finds things completely different than the way Marty described him. Jeff can only conclude that Marty is hallucinating -- if that's possible for a ghost.
The highlight of the mindreading act of Fernandez and Abel is a blank cartridge placed in a revolver, then the correct chamber containing the cartidge predicted. This time, the cartridge isn't blank, it's real--and Abel fatally shoots his partner. Jeff, who was in attendance at the fatal show, gets a job as a solo mind-reader (thanks to his ghostly partner) to try to find out who was responsible for putting the real bullet in Abel's gun.
Jeff and Jean are hired to be escorts for various contestants in the Miss World beauty competition. Jeff draws the assignment of escorting Miss London, who is working with a man to steal some documents from the Hall of Records that will prove he is the rightful heir to the throne.
Paul Kirstner, a long-time underworld figure, arrives in London from America with a good deal of baggage in tow, including the ghost of his one-time partner, Bugsy Spanio. Kirstner, responsible for Bugsy's death 35 years earlier, hires Jeff to look after his daughter. Bugsy, meanwhile, tries to force Marty into getting Jeff to kill Kirstner in retaliation for what Kirstner did to Bugsy.
A criminal gang is feeding Marty false information (which he gives to Jeff, who in turn tells the police) to keep the cops away from real burglary targets. Once Marty has served his purpose, he will be exorcised by a clairvoyant gang member, who has been working on Jean's emotions to make her an unwitting participant in the plan to dispose of Marty.
For once, business is booming at the Randall and Hopkirk office. An insurance company hires Jeff to investigate the disappearance of a shipment of diamonds. At the same time a real estate agent hires Jeff to sort out rumors about Merston Manor, a house scheduled to go on the real estate market, being haunted.
Marty, in a particularly good mood, materializes on Jeff's desk one Monday morning. Jeff, however, fails to respond to Marty's voice. He doesn't even look at Marty. What's worse, he acts as though he couldn't care less about the sudden disappearance of his ghostly partner. Jeff seems to have two things on his mind: his new case involving the leaking of stock market information at the Towler Corporation, and Jean's figure. Marty becomes more alarmed when he witnesses Jeff kill a Towler Corporation executive.
Marty's aunt Clara has spent five years perfecting a ""system"" that guarantees a win at roulette. She hires Jeff and Jean to escort her to Monte Carlo as ""bodyguards"". They have plenty of work to do: three different groups of people seem to be after Aunt Clara and her ""system"".
Self-proclaimed ""ghost hunter"" James McAllister offers Jeff half the money he's making from millionaire Kim Wentworth to prove that her home, Crake Castle, is not haunted, even though McAllister claims to have seen the ghost. After Jeff and Marty join the case, McAllister turns up murdered.
Julia Fenwick asks Jeff to find her missing aunt. The aunt stayed to herself, only going out once a week to seances. Jeff asks Jean to pose as a wealthy widow trying to contact her departed husband ""John"" to see what connection there is between the seances and a financial consultant who was the last person to see Julia's aunt.
Bird lover Mrs. Wentworth Howe left her estate in trust, stipulating in her will that the money would not be distributed among any relatives as long as a bird remained alive in the aviary at the Howe mansion. When one of the birds is shot dead, estate trust attorney Laverick hires Jeff to investigate. When Jeff shows up at the mansion, the list of possible suspects grows shorter and shorter as the relatives are murdered one by one.
In the bathroom, fresh from her shower, Jean Hopkirk hears a male voice call her name. She finds a strange man lying on the sofa, newspaper in one hand and drink in the other, making himself very much at home. The strange man has an even stranger claim: he is Marty Hopkirk, reincarnated.
Calvin P. Bream is a cocky con artist, until his boast of an unlimited number of bearer bonds puts him in contact with a ruthless criminal by the name of Miklos Corri. Desperate to save his own neck, Bream says he's only the ""middle man"". Bream finds the name of a detective agency in the classifieds and heads there to throw Corri off his own trail. The agency he picks is Randall and Hopkirk.
Marty's headstone is knocked over, and Jeff is knocked out by a man dressed in 18th century attire when he goes to investigate. What does it have to do with a wealthy man and his agoraphobic son who live in a mansion next to the cemetery?
Jeff and Jean see a man leave their car after a party. The man left a body in the back seat. The case is open-and-shut, with both Jeff and Jean identifying the man in a police line-up, but the day before they are to testify Jean is kidnapped by the man's mother. Marty tries to find her, but every time he concentrates on Jean's location he ends up at their apartment.
Jeff is hired -- much against his will -- by two rival criminals. The task seems simple: go to Glasgow, pick up an item worth £10,000, and bring it back to London. Things become complicated when the ""item"" turns out to be the girlfriend of one of the criminals -- and she promptly takes off, leaving Jeff to take the blame for a double-cross.
Just at a time when Jeff could use some money an old friend shows up with a money-making proposition. Not so much money ""making"" as money ""taking"": he suggests to Jeff that they steal old notes that are en route to the incinerator. Jeff watches from a distance to see if his friend shows up and ends up in jail charged with the crime when the police prove he was in the vicinity and his alibi falls through.
Thanks to a broken leg, Jeff is a captive audience (in a hospital bed) while Marty recounts the greatest case the Randall and Hopkirk office ever handled, involving MI5, spies, and a list of British agents Marty was tricked into stealing from a defense minister's home.
Joshua Crackan is having a birthday party, gathering his relatives from various parts of the world. The assignment for Jeff is easy: he has to make certain that only those with invitations enter Crackan's property. Before the party Jeff and Marty have an argument and Jeff asks Marty to leave him alone. Now that the party guests are dying, Jeff is having second thoughts about handling the case without his ghostly partner.
Jeff is on a ""cheating husband"" stake-out, his least favorite thing. A man is shot outside a club near where he is parked. After Jeff finishes talking to the police he returns to his office, where he meets a woman waiting to hire him. She claims the murdered man was mistaken for her husband. Jeff takes the case, then finds himself accused of murder when the woman's husband is found dead.
Eric Jansen breaks out of the psychiatric unit of prison shortly before the anniversary of his arrest with vengeance on his mind. One problem, though: the man responsible for putting him behind bars, Marty Hopkirk, is already dead. Jansen decides that someone named ""Hopkirk"" is going to pay, so he sets his sights on killing Marty's widow, Jean.
Jeff is charged with a break-in at an electronics company. He's obviously been set up, given that the first thing he saw when he walked into his apartment after an all-nighter was a nun telling him the electronics company was a convent. Everything has been made to look totally unbelievable, which is why Jeff has difficulty trying to convince the police to give him time to prove he has been framed.
Marty sees a funeral near his grave. Based on what he overhears he's certain that his ""new neighbor"" suffered the same fate he did: a murder made to look like an accident. Jeff has an appointment with a prospective client and is unwilling to do something for Marty for free, so Marty must trick him into looking into the death.
