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.

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

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.

Robin's friend and aspiring writer Betty comes to the estate for a quiet place to overcome her case of writer's block as she works on her mystery adventure novel. In short order, she convinces Higgins to hand over the keys to the Ferrari and to relocate Thomas to the main house while she works in the guest house, much to Thomas's extreme displeasure. After Thomas is forced by a lack of transportation to let her observe his investigation of a hit-and-run insurance case, she begins to churn out a story inspired by Thomas's case, featuring Thomas as dashing international investigator Sebastian Sabre, Higgins as his faithful servant Boris, T.C. as his pilot Winston, Rick as club owner Swift, and herself as Sabre's glamorous girlfriend Collette. Her wild imagination unwittingly breaks open Thomas's investigation and plunges them both into danger.

After Agatha and her friends are swindled out of $100,000 by a shady investment company, she hires Thomas to track down the crooks and retrieve their money; Higgins's distant cousin arrives to marry the heir of a local socialite, and turns out to be a punk rocker in need of a drastic makeover before she can be accepted into society, giving Higgins and Agatha only a few days to play Pygmalion and turn his common cousin into a proper lady before the wedding.

Magnum has been hired by country musician Lacy Fletcher to unearth five songs written over twenty-five years ago by legendary singer George Lee Jessup shortly before his death. Jessup wrote the songs for his girl-friend, but when Thomas tracks her down, the woman denies ever knowing Jessup or any knowledge of the songs, and it seems that Lacy is not the only person searching for the songs...

Thomas is hired by Jack Damon, Carol's Uncle, to find his seventeen-year-old daughter Becky, who has been missing for two months, after leaving home against his wishes to pursue a career as a dancer. After a little investigating, Thomas finds that the last person known to have seen the girl was her former boyfriend Darryl Jacobs – who is now behind bars after being involved in a stolen guns deal that went bad. Magnum decides the only way to find out from Jacobs what happened to Becky is to go undercover as a convict at the prison farm where Jacobs is being detained...

A supposed wealthy teenage ranch owner hires Magnum for $100,000 to stop a dangerous group of cattle rustlers on Hawaii's big island, that will soon force his ranch into bankruptcy if many more cattle are taken. But Magnum finds that his employer isn't quite all he makes out to be and will go to any extent to protect his cattle, and the neighbouring ranch owner implicated of being behind the cattle theft is a business partner of Robin Masters'...

Thomas is more than a little taken aback when Higgins suddenly leaves his position on the Estate, after seemingly being found out by Robin Masters for stealing large amounts of money and valuable art objects. As Higgins' replacement, a rather dizzy actress, arrives to take his place on the Estate, Thomas investigates the mystery surrounding his abrupt departure...

Higgins is preparing for a prestigious chess tournament, and the arrival of an important visiting dignitary, the President of Costa Del Rosa, who will be attending it. There have been threats made on the President's life, and Magnum is asked to apprehend those responsible. But both Magnum and Higgins have even more to contend with when yet another of Higgins' illegitimate half-brothers, the flamboyant Don Luis Mongueo, the last of the royal line of Costa Del Rosa, makes an appearance on the scene. But with the plots to assassinate the President, Don Luis emerges as the prime suspect...

When Higgins unwittingly causes Magnum to wipe an expensive computer game loaned from T.C., and Thomas accidentally erases a chunk of Higgins' memoirs stored on the computer, a feud breaks out between the pair. Magnum is trying to break a major, crooked gambling ring that is operating on the islands, but it is none too easy while in the middle of the falling out with Higgins, which starts out with each trying to out-do the other with ""eye-for-an-eye"" practical jokes, and quickly develops into a particularly bitter battle of one-upmanship...

Higgins is driving visiting Pamela Bates - an old editor friend of Robin Master's - along with her friend, and her secretary, to the Estate, when someone tries to run them off the road and over a cliff. But Pamela insists on waiting for her own mainland investigator friend famous crime mystery writer Jessica Fletcher.

The crossover continues on Murder, She Wrote S03E08 Magnum on Ice (II).

Survival expert Higgins goes on a survivalist trip on an island with a small group of young offenders as part of a programme aimed at improving their self-esteem. But he has to call upon his survival skills much more than expected when a planned escape by the boys doesn't go as intended and sees the leader of the trip shot dead, leaving Higgins being hunted down on the island by the three young convicts... Back on the Estate, Magnum, with Rick and T.C., plans to take advantage of Higgins' absence by holding a big party, but the get-together seems ill-fated from the start...

T.C. is giving Thomas helicopter flying lessons when he is shot. With T.C. hospitalised, Mac appears on the scene, ""volunteering"" his services to stand in running Island Hoppers. But it seems that the assailants were actually gunning for Magnum, after a week previously, somebody impersonated him when trying to blackmail a Mexican Mafia King. With Magnum marked as a dead man as a result of the mistaken identity, confusion and chaos reigns, and as per usual, Mac 'just happens' to be in the middle of it all...

After a run of bad luck, including being fired from a case investigating embezzlement, owing Rick money, and letting T.C.'s junior baseball team down, Magnum decides to get away from it all by going on a solo mountain-climbing hike, without telling anyone where he is headed. But on the climb, already suffering from a centipede bite, he discovers the wreckage of an old World War II fighter plane, only for an accident to case him to become trapped under it. With his legs caught under the wreckage, and no-one knowing he is even there in the first place, Thomas is forced to review his mortality, his values, and the case which he was fired from...

Cynthia Farrell comes to Hawaii to see Magnum, and to see if the two can recapture the magic of their relationship in L.A., but when Magnum is hired by an adult film star, he spends more time looking after his case than he does Cynthia.

Thomas is delivering key evidence at the courthouse when he is shot in the parking lot by a hitman. In a coma, Thomas needs to save Michelle from some killers, and meets up with the ghost of his former Naval buddy Mac. Then he bids each of his friends a personal goodbye, in case he doesn't make it.

Still healing from his near-death ordeal and experiencing Mac's ghost, Thomas is surprised when Higgins hands over his estate duties to be able to take some time off; in a bizarre twist, Higgins starts living the fast and loose life, wears loud Hawaiian shirts and becomes irresponsible, while Thomas assumes Higgins' stuffy personality and ways, going so far as to wear a suit; curious about Higgins's sudden change in personality, Thomas investigates and discovers that Higgins is seeing a woman on the sly, and becomes concerned that she is taking Higgins for a ride.

Higgins wants Magnum to find the ""Lost Art of the Ancients"", a priceless ancient scroll from a fallen civilization, that could spell disaster if it should ever fall into the wrong hands. But the search for the long-lost artefact is laced with abduction, danger and deceit, as an old flame of Magnum's, a foreign double agent, and a ruthless rival of Higgins' are all out to find the scroll as well, and the dangerous path to unearthing it could ultimately lead to death...