It's the 4th of July, and Higgins is playing in a polo match, T.C. is taking his baseball team to a professional game, Rick is out with his latest girlfriend on the King Kamehameha II yacht, and Thomas is observing his annual Independence Day tradition of a solitary day at sea -- until his surf ski is capsized in the wake of a recklessly driven speedboat and set adrift. Carried off further and further away from land by the powerful Molokai Channel, Thomas relies on childhood memories of his father's training him to tread water to keep himself afloat as he struggles desperately to stay above water and fend off a menacing shark. As the hours pass, Higgins, T.C. and Rick each have a sixth sense that Thomas is in trouble, and they team up to rescue him. Thomas approaches the twenty-four hour mark in the water recalling his wedding to Michelle and his father's burial -- on July 4, 1951 -- as Higgins scoops him out of the ocean, with T.C. in his helicopter overhead and Rick on the yacht near

When Thomas and old-school private eye Luther Gillis encounter each other in a seedy hotel room with a dead drug dealer as they separately investigate the whereabouts of Nancy Perkins, a St. Louis runaway, they decide to team up to find the missing girl after the police burst into the room and arrest them on suspicion of murder. After Thomas finds out that Luther is actually Nancy's father, Higgins poses the question that leads Thomas to solving the drug dealer's murder and reuniting father and daughter.

Thomas's attempt to impress a new girlfriend by taking hang-gliding lessons lands him in the hospital with a broken shoulder; Rick bails out his childhood friend Waldo Norris on a breaking and entering charge, but Waldo insists that he's an insurance claims investigator trying to prove that a valuable figurine that was reported as stolen is still in the possession of its owners; after they get shot at by a man ransacking Waldo's hotel room, Rick does a little digging and discovers that his friend has been lying to him, and has also stolen one of Higgins's sketches by conning him into thinking that he's a representative of the Smithsonian; Waldo then confesses that he's really a CIA operative and the figurine contains sensitive information; after some further checking, Rick is able to verify his story, and Higgins and T.C. agree to help Waldo and Rick -- that is, until Thomas does even more checking and discovers that Waldo was fired from the CIA years before; Waldo then claims that he

Under protest, Thomas agrees to chaperone Rick's visiting 22-year-old sister Wendy while Rick works at the club, and quickly discovers that she's not the convent school innocent her brother thinks she is; after Wendy gives him the slip, Thomas scours the city searching for her and is devastated when the police find her murdered in an alley; when Rick blames Wendy's death on Thomas, the relationship between the men is dealt a severe blow, and Thomas vows to find her killers, even though Rick is determined to do the job himself with the help of information and an Uzi provided by Ice Pick; Thomas and T.C. follow Rick to Molokai, where the murderers are killed in a fiery crash as they are pursued by the three friends; to spare Rick's feelings and ease his grief, Thomas tells his friend the convincing lie that Wendy was a good kid who was just in the wrong place in the wrong time, instead of the truth that she was a drug mule murdered by the men for whom she worked.

Thomas's assignment to investigate the series of thefts that have been plaguing a chain of convenience stores brings him in contact with two elderly sisters who have a car accident in front of one of the stores; when Thomas learns that they are at the mercy of a slumlord about to evict them and the other elderly impoverished residents of their dilapidated boarding house, he springs into action and convinces Higgins to host a charity bingo game to raise enough money for them to buy the house; after the sisters disappear with the proceeds of the game, Thomas discovers a link between the women and the robberies.

Magnum is preparing for this year's big surf-ski race, while in a chance meeting, Higgins meets Lady Wilkerson, an English Duchess whom he has been smitten with after two brief encounters years before (which she does not identify him from). She is in the islands incognito, and must sell some valuable family jewels after being strapped financially, but Higgins ends up saving her from two men who accost her in an alleyway. As smitten with the woman as ever, Higgins offers her a job on the Estate, and plans to reveal his feelings towards her via a letter, but due to a mix-up and misunderstanding, she assumes the letter to be from Thomas instead, whom it becomes apparent she has eyes for, blind to Higgins' desires. But these confused matters of the heart seem trivial when the Duchess is kidnapped by the two men who had made a grab for her previously...

After a high stakes game of poker against high-flying men's magazine publisher Buzz Benoit, Robin Masters wagers use of his Hawaiian Estate for a year, to be decided in a softball game between the King Kamehameha Club softball team and a team assembled by Buzz. Higgins, upon learning that Buzz intends to use him as a butler, plans to leave the Estate if the King Kamehameha team loose, but Magnum is less worried, looking forward to a luxurious year on the Estate surrounded by Buzz's beautiful glamour models - until he learns that as a result of Buzz's planned changes, there will be no place for him on the Estate either. Meanwhile, Magnum is working for one of the members of the softball team, trying to find her missing ex-husband who owes her alimony money, but things look gloomy for Higgins and Magnum's future on the Estate when they find that ruthless Buzz has hired a professional team of players. But it looks as if Magnum's team may have a glimmer of hope when it turns out that t

Eager to attend a ten day Tiger home stand, Thomas convinces T.C. to finance the expenses for the trip in exchange for Thomas's collecting the $10,000 fee that T.C. was stiffed by a man from Motor City, and handing T.C. the keys to the Ferrari while he's gone; while tooling around in the car, T.C. literally runs into Leon, a fellow Vietnam vet turned professional boxer, and his young daughter Ima; feeling responsible for Leon's broken collarbone, T.C. offers to take his place in the two remaining club fights Leon needs to win to claim the purse that will lift him out of his financial difficulties; T.C.'s eagerness to substitute for Leon dims when he learns that these are bare-knuckle, no-holds-barred fights, and that there are many more than two fights he must win before collecting Leon's purse from a promoter who turns out to be crooked; back in Detroit, Thomas is having more difficulty than he anticipated collecting T.C.'s debt and attending a game, until two strangers in a bar take

With help from T.C. and Rick, Thomas is preparing a surprise construction for Higgins' upcoming birthday, when they wager a bet that a presenter on a local radio station is the same woman that was a much-loved presenter for the troops in Saigon. Thomas goes to the radio station to try and confirm whether it is indeed the same person or not and finds that she is the self same person, but now is being terrorised by malicious death threats phoned in to the station. The woman also has been searching for her lover, with whom she lost contact with 12 years ago during the war, when the man was accused of theft and disappeared, and asks Thomas' help in finally finding him...

It is Christmas Eve, and T.C. is flying Rick to judge a beauty pageant, Thomas to play Santa Claus at a charity event, and Higgins to deliver a payroll to some of Robin Master’s workers, before he goes home to spend Christmas with his family in New Orleans. But the helicopter suddenly develops problems and they are forced to make an emergency landing on nearby deserted Frenchman’s Island. The remote isle is used by the Navy for shelling manoeuvres – and as the foursome try to find a way to safely make it off of the island, hopefully in time for Christmas, a Naval Captain, unaware of their presence, is hell-bent on upping his ship’s target practice scores, and a shelling on the island is imminent...

The Crown Prince of Jororo accompanies his teammates when their baseball team comes to Hawaii to play T.C.'s team, and takes up residence on the estate as a safety precaution against the death threats he's received. Thomas strikes up a friendship with the boy, who is eager to escape the ever watchful eyes of his bodyguards and governess, and who has a nasty habit of escaping whenever the opportunity presents. When terrorists attempt to assassinate the young prince, Thomas goes all out to protect him from his father's enemies, who turn out to be closer than anyone imagined.

Higgins sustains a concussion hours before the guests are to arrive for a charity gala at the estate, and begins to lapse in and out of believing he is famed Shakespearean actor Sir Fearing Pangborn; Thomas enlists Rick and T.C. to cover security for the event while he takes up the slack left by Higgins's injury, and has his hands full when all the guests, dressed as famous characters of the 1920s in keeping with the gala's "Great Gatsby" theme, begin to arrive hours ahead of the appointed time; a crime wave breaks out when someone dressed in the same Charlie Chaplin costume as Rick is seen fleeing from a guest's room moments before thousands of dollars in cash and jewelry are reported missing and an addled Higgins is the prime suspect in the murder of one of the guests; as he begins to realize that people and events are not what they seem, Thomas takes his inspiration from the six-hour First Annual KHKU Agatha Christie Film Festival he viewed the previous night to solve the crimes a

Thomas is forced to forgo his much-anticipated trip to the Army-Navy game and a reunion with his 1967 championship teammates when Rick, T.C., and Higgins guilt him into remaining in Hawaii and helping Carol save her career by nabbing one of the largest drug distributors in the islands.

Magnum is hired by a bank teller from Robin Master's bank to investigate her father, world-famous counterfeiter Herbie "Rembrandt" Norton, who has recently been released from penitentiary and whom she suspects has returned to counterfeiting after coming into large amounts of money since his release. Before starting investigations, the pair go to retrieve a book for Higgins from a safe deposit box at the bank, but the woman accidentally locks them in the vault, and with the time-release door and a slowly dwindling air supply, the pair are trapped inside... Meanwhile, spurred on by getting the potentially lucrative case, Thomas had promised to take Rick and T.C. for a luxurious weekend away, but unaware that Magnum is trapped in the vault at the bank, they are annoyed to presume that he has gone without them, and - with T.C. having problems with his girl-friend - they decide to hold a party at Magnum's guest house - which does not go down well with Higgins, who is holding a meditation we

T.C. drags a reluctant Magnum along to the newly-opened club of an old flame – a beautiful jazz singer who he fell in love with in Vietnam – who has just moved to the islands. Thomas doesn’t dislike the woman, but insists to T.C. that trouble always follows her wherever she goes – but T.C., still infatuated with the girl, refuses to listen. But sure enough, the woman is in trouble, searching for a man who owes her $18,000 and mixed up with Detroit drug dealers that are out to kill her...

St. Louis-based Private Investigator Luther Gillis returns to Hawaii, accompanied by his secretary Blanche, to attend the annual Private Investigators convention, where Luther has been informed he will receive an award. But while Luther and Magnum attend the convention, Higgins and Blanche are kidnapped from the Estate, and the two P.I.s find themselves having to join forces once again...

While Higgins is preparing to direct a selection of pieces from Gilbert and Sullivan's 'The Mikado' to be staged at the Estate, Thomas is hired by an attractive young woman to find her missing brother, who she claims has disappeared after joining a religious cult. The girl has musical experience and agrees to help out with rehearsals of Higgins' production while Thomas seeks out her missing brother, but Thomas uncovers more than he expects and finds himself dealing with a radical political group and an assassination attempt revolving around a visiting cast member of Higgins' production...

As he continues his memoirs, Higgins recalls the spring of 1976, when old friend from Sandhurst, David Worth, arrives in Honolulu to attend the funeral of a mutual friend. Insisting that his name is Stanley Sigerson, Sherlock Holmes's most frequently used alias, and that Higgins is Watson, Holmes's faithful friend, David firmly believes that their friend was murdered by none other than Moriarty, Sherlock Holmes's arch-nemesis. In the name of friendship, Higgins indulges David in his investigation, even though his suspicion that David is insane is borne out, and is shocked to discover that David's assertion that their friend was murdered was absolutely correct.

Carol persuades Magnum to do some surveillance work for a case she's working on, but the job leads to a high-speed car chase which ends in near tragedy when Thomas returns gun-fire only for it to accidentally hit another car, causing it to crash and severely injuring the young woman driving it. With the girl, Emily, facing never being able to walk again, Thomas is racked with guilt, and befriends her, determined to help her regain use of her legs. But unbeknown to Magnum, the two thugs that he was pursuing are still tailing him...

Thomas remembers his first client and his first case as a Private Investigator in 1979, when he was hired by a surfing champion who was being threatened, and was on the verge of entering into a serious relationship with her. The memories are brought on when now, in the present day, she calls for his help once again, to protect her and her young daughter after she again begins receiving threats. But events of present day seem destined to parallel the events of 1979...

When the King Kamehameha Club is robbed late one evening by thieves wearing animal masks to conceal their identities, Lt. Tanaka becomes so frustrated by the inability of Higgins, Rick and T.C. to agree about what transpired that he enlists Thomas's help in getting their stories straight. Since the men tell widely divergent versions of the night's events designed to show themselves in the best possible light as the evening's hero, it takes Thomas quite some time and effort to uncover the inside man at the club who tipped off the robbers.