As Magnum helps a woman find her missing cousin, Sergei, a Russian fugitive, Higgins' and Kumu's lives are put in danger when they get too close to the case.

Magnum and Higgins are shocked when one of her former MI6 teammates shows up with a fresh gunshot wound and news that "the Viper," the assassin who killed Higgins' true love, has surfaced on Oahu. Also, while Magnum's busy helping Higgins track down the Viper, he asks Rick to take over a case, a task that Rick relishes until it goes sideways.

Magnum discovers that his ex, who landed him and his friends in a POW camp, is on the island and planing to steal a fortune in gold.

When the Dobermans dig up what appears to be a human bone, Magnum and Higgins help Katsumoto investigate the murder of his former mentor.

Hannah, Magnum's ex and the fugitive responsible for him and his SEAL team brothers being POWs for years, shows up at his home with a gunshot wound, and wanting his help to find her missing CIA father whose life is in danger. Also, Magnum makes an earnest proposition to Higgins for her to become his private investigations partner.

While Magnum waits for Higgins to decide if she’ll officially become his partner, he takes the case of a missing wife who witnessed her boss’ murder during a bank robbery. Also, Rick makes a big decision about his career.

On Halloween, Magnum and Jin search for a skip-tracer at a Halloween bash, just as Katsumoto tracks an escaped killer who is headed to that same party. Also, Higgins and Rick must spend the night at an allegedly haunted house in order to debunk the notion, and TC and Kumu protect a sacred Hawaiian burial ground.

Magnum and Higgins help Rick when his father figure, Icepick , just out of prison and battling terminal cancer, gets double crossed on one last score. Also, Magnum makes one last bold move to help Higgins stay in the country.

Magnum and Higgins’ new clients hire them to find their missing brother last seen on the island, leading to Higgins being shot and TC abducted when a secret case of revenge is revealed during the investigation. Also, Higgins (and Magnum!) are surprised when her surgeon asks her out.

Higgins job to repo a plane goes south when she and Magnum discover that it was being used by a drug cartel who opens fire at the plane, forcing them to crash land in the jungle, where they find a stowaway on board with a target on his back. Also, Higgins has her first date with Dr. Ethan Shah.

As Magnum tries to keep his new relationship a secret and handles cases solo while Higgins is away, he is tasked with finding a single mom being hunted by two very dangerous gangs. Also, while Higgins is abroad with Ethan, a misstep forces her to assist an old employer, and Rick gets some bad, but also very, very good news.

Higgins is in a complicated situation when 12-year-old Ella asks for help cracking a triple homicide that involves her mother. Also, Magnum bonds with Bob, a terminally ill veteran, while volunteering with the No Veteran Dies Alone Program and attempts to reunite him with his estranged family.

Magnum and Higgins are sent on a deadly mission inside a high security correctional center when a ruthless criminal abducts Katsumoto and his ex-wife, Beth.

Magnum and Higgins weigh the risks of pursuing a relationship. Rick juggles being a new father and running La Mariana. Katsumoto tries to figure out his next move.

Magnum and Higgins go undercover as lifeguards in order to solve a murder. Jin Jeong wins a storage unit auction and enlists the help of Rick and TC upon finding a stolen military medal.