Vain, arrogant, fading movie star Anne Marion calls a press conference to announce plans for an autobiography. Then, a bomb goes off. The actress hires Intertect and, after a look at available operatives, decides Mannix is her man. The assumption was the bomb was a publicity stunt. Mannix visits one of Anne's ex-husbands, who performs special effects work. The bomb was supposed to be a dud. Mannix is convinced that someone wants to do the actress harm. She has three suspicious ex-husbands, one of whom is an arrogant hothead, the other who employs shady and violent ...
Three people are brutally murdered. The evidence points to David Tate. When a potential client approaches Intertect about trying to prove Tate innocent, Mannix agrees to take the case. The private detective finds himself caught in between the police and whoever is framing Tate.
An Intertect security chief apparently has been killed while driving. At almost the same time, an explosion at an Intertect client has killed a technician and destroyed a new laser device. Mannix suddenly is thrust into the case while Intertect chief Wickersham begins acting erratically, the side effect of medication for an infection. Mannix concludes that the Intertect security man may have sold out, along with somebody at the client. Mannix later finds that the Intertect security man is still alive. Mannix, though, isn't having an easy time. The Intertect chief's ...
A doctor for Intertect arrives and treats Mannix's injuries. The physician also informs Mannix that Lew Wickersham has been taking steroids prescribed to treat an infection. That's why Wickersham's behavior has been erratic. Mannix and Wickersham separately probe the conspiracy that has left three people dead. If they aren't careful, the death toll will rise again.
Mannix quits Intertect after Wickersham agrees to accept a case from suspected gang boss Aram Karmalis. Striking out on his own, Mannix agrees to help a young woman. But it turns out that Mannix is now working a different angle of the same case.
Down on his luck Bernie Farmer is married to Rose, who's the sister of a mobster. When the criminal is about to be paroled, Bernie decides to open a box the mobster had entrusted to him. But the box is empty. Mannix and Intertect boss Wickersham, against their better judgment, opt to help Bernie find what was in the box. Mannix sets himself up as a target, letting people think he has the box and its contents. The detective will endure multiple beatings as he tries to solve the case.
Mannix is hired to investigate a supposed miracle that has occurred on land owned by a mobster. When a man turns up dead and the mobster's thugs beat on Mannix, the miracle's legitimacy seems highly doubtful. But the motives for faking the miracle are murky, at best. The mobster doesn't stand to gain financially. But Mannix is determined to find the truth.
A man named Spinelli shows up severely injured at a hospital emergency room, asking for Mannix. The detective arrives. He's never seen Spinelli before in his life. From this odd beginning, Mannix will be beaten, suspended by Intertect and shot at with a spear gun as he tries to solve a mystery extending back more than 20 years.
A mystery writer finds a suicide note in his home -- supposedly written by himself. The author specializes in stories that are based on actual cases and he's working on a new novel taken from an unsolved murder. Mannix tries to protect the author, but the writer insists on trying to work on the case. This complicates things greatly for Mannix. In addition, Mannix must deal with a collection of shady characters as he tries to find the would-be killer.
Wealthy art collector Calvin Norris, while visiting an artist friend, sees a woman modeling for a sculptor in the studio across the alley. Norris is shocked. The woman looks exactly like the subject of a Renoir painting he owns. He tries to get to the woman but she is gone before he can get to the sculptor's studio. Norris hires Mannix and Intertect to track the woman down. Norris is more interested in her than the possibility his painting is a forgery. Mannix, relying more on legwork than Intertect's computers, finds the woman, who, it turns out is really an artist ...
Mannix has finally quit the detective agency for good and is out on his own. The second season begins when Mannix encounters a deaf woman who has witnesses a criminal talking at a pay telephone, discussing a kidnapping. She has made out certain key words because of her ability to read lips. Mannix gets her to remember enough of the conversation so that he can proceed in his investigation. The detective finds a rich man who says his wife has been kidnapped. It turns out the rich man has really hired the criminal to kill his wife and make it look like a kidnapping - and...
Charlie Anderson, a former policeman who once saved Joe Mannix's life, hires the detective when his wife has run off. The wife is somebody else Charlie met while a policeman - a one-time prostitute who went straight. Charlie now is a security guard at a air cargo facility. One test run has gone badly, thanks to Charlie being suspicious of hoods. Mannix, after being beaten by hoods, learns in the hospital that Charlie and his wife are together again and that he's off the case. Mannix is determined to find the truth.
Joey, a young man who helped Mannix out on a previous case, shows up in the detective's apartment, injured from a beating. Mannix hides Joey and gets a call out to the police just before the apartment is overrun by hoods. Mannix is beaten and when he wakes up, Joey is gone. As the detective follows the trail, an organized front tries to buy Mannix off with a trip to Hong Kong on a supposed case. It turns out that the target of all this is a judge, whose daughter is in love with Joey. The syndicate wants to pressure the judge to move the site of a trial for an ...
Mannix has taken up a new hobby, amateur auto racing. While participating in a race, another racer is killed when his car seems to lose control and goes over an embankment. The widow of the dead racer contacts Mannix, wanting him to investigate, believing the death was a homicide. As Mannix proceeds, it's clear federal authorities have much more interest in what seems like a routine case.
A recently paroled ex-con is killed by a hit-and-run driver. The man had a matchbook with Mannix's phone number in it. Mannix tries to find out why the ex-con was killed. As his investigation, proceeds, Mannix discovers a plot -- and the detective may be its next victim.
An author, preparing a book about a well-publicized murder case in a small California city, is killed when his yacht blows up. The author's publisher hires Mannix to find the book's last chapter, which is said to solve the murder case. Mannix encounters a hostile police chief who wants to chase him off and an oil-company executive wanting to buy him off. Mannix refuses the bribe and endures a beating by the corrupt police chief but remains committed to finding the missing chapter.
Mannix is hired by successful heart surgeon Cameron McKenzie, whose young son was apparently kidnapped by someone who looked exactly like the doctor, driving a car that also looked exactly like the doctor's car. Mannix agrees to look into the kidnapping if the police will be notified once Mannix runs into a dead end. But then Dr. McKenzie confesses something he has held back: the kidnappers don't want a ransom -- they want McKenzie to let one of his patients die in exchange for the boy's life.
Mannix is hired by a secretive multi-millionaire to find his ex-wife. At least that's what he has been told when he takes the job. The ex-wife, a prominent singer, supposedly committed suicide but has been seen since her supposed death. As Mannix pursues the trail, it becomes clear he hasn't been given the entire story. The lives of Mannix and the ex-wife may soon come to an end unless the detective solves the mystery.
Mannix seeks to help a man released from prison after seven years. The detective helped send him to prison. This occurred when Mannix was on his first case and, as he remarks, "I was green" then. After the man's release, people start dying at his former employer. Mannix must probe two possibilities: whether the man was framed seven years ago or whether he is seeking revenge now.
A young man named Clifton Ross (Mart Hulswit), the son of an aircraft magnate (David Brian), awakens after a night of drinking to find a bandage on his hand, blood on his shirt, and fragmentary memories of a fight with his girlfriend Doreen (Janaire). Ross learns that Doreen never came home the night before, and fears that his memories suggest that their evening together ended violently. Ross decides to hire Mannix to reconstruct the previous 24 hours of his life.
Mannix is traveling to Santa Barbara by helicopter when he sees a woman apparently being strangled on a penthouse terrace. When Mannix and the police investigate, however, they are assured by the occupants of the penthouse apartment that nothing has happened. Mannix refuses to accept this and launches his own investigation.
Mannix is brought to a small town in New Mexico on a ruse, and then subpoenaed to testify as an expert witness for the prosecution in a murder trial. After his testimony, the woman representing the defendant announces she wants to resign as defense counsel. Mannix decides that instead of returning to Los Angeles, he'll remain to aid the defense, encouraging the defendant's attorney to stick with the case while he tries to dig up new information that could free her client.