While Kono Kalakaua (Grace Park) and Adam Noshimuri (Ian Anthony Dale) are away on their honeymoon, the remainder of Five-0 investigate the murder of a researcher who was investigating a piracy incident in Oahu in 1884, where artifacts were stolen from King Kalākaua that has since become a local legend. After the killer steals a painting from a museum, the only recovered artifact, he takes a hostage and disappears. It is revealed that the hostage and the killer are working together as modern day pirates. Steve McGarrett (Alex O'Loughlin) tells Danny (Scott Caan) that he is considering proposing to Catherine (Michelle Borth). Meanwhile, Gabriel Waincroft (Christopher Sean) holds Kono and Adam captive demanding Adam relinquish his assets. After Adam eventually gives in, he and Gabriel go to a bank. Kono manages to escape to stop Gabriel, but Gabriel wounds Adam to ensure his escape. With Adam's money gone, he has no means to pay the Yakuza to keep them alive.

A bomb squad is attacked following a tip of a briefcase bomb. When another briefcase is left abandoned at a local news station, Five-0 discovers that it contains a flash drive containing a ransom to release serial arsonist Jason Duclair (Randy Couture) or another bomb will detonate. The bomb was set by Jason's believed protégé however it's later revealed he only wanted to kill Jason so he could have more fame. Meanwhile, Adam is released from hospital and recovers with Kono, but they are quickly stalked by the Yakuza. Kono later confronts their car to see the gangsters dead.

Diver and artist Ben Lahineha is found shot in the ocean while gathering lost objects on coral. Five-0 find that ballistics match the murder of a John Doe weeks before. Meanwhile, Danny gets his nephew Eric Russo (Andrew Lawrence) a job with the crime lab, and Jerry is given his own office in the basement. McGarrett prepares to propose to Catherine, but she dumps him with a lie.

While Danny is recovering from a bone marrow transplant to his son, the rest of the team investigate the death of Harrison Crane (Keith A. Kirksey), a reclusive millionaire who disappeared 20 years ago, who was found in the jungle with Moesley's counterfeit money. Meanwhile, Adam is captured by Tom Bishop (Steve Bastoni) of the Yakuza and is forced to torture Aaron James for Gabriel Waincroft's location. When Aaron is revealed to not know anything, Bishop executes him.

Lawyer Kevin Harper is killed in front of his wife and daughter by a hitman riding a motorcycle during a deposition looking into his embezzlement of company funds. Harper was ready to admit that he embezzled money to pay for protection money for the Samoan mafia, and the team believe they were silencing him. Meanwhile, McGarrett receives a mysterious phone call from an unknown location, thinking it might be Catherine. Also, the team enters in a Tough Mudder race.

On Halloween, Five-0 investigate the discovery of a briefcase with a number of similar looking female corpses with missing body parts. The team suspect the killer to be Mark Sheppard (Michael Graziadei), an ex-con obsessed with the occult, and one of the victim's ex-boyfriend, but is ruled out when he is found attempting suicide. Meanwhile, a blood bank is robbed by a local gang while Jerry gives blood. Chin and Kono find the thieves took a rare blood type and start looking at pet clinics suspecting the thieves are hiding in one. Danny discovers that Grace went to a party in the North Shore and comes to pick her up.

McGarrett goes on a first date with Lynn (Sarah Carter), and they go exploring a small island together. After the two come across an abandoned plane in the jungle, they are ambushed by a gunman. After McGarrett subdues him, he and Lynn explore the plane to discover that it was a prison transport plane that held Dennis Logan (James C. Burns), a crime boss from Boston who was believed to have been killed in a plane crash four years ago. Meanwhile, Chin, Kono, and Grover attend a college football game where the star player Jesse Frontera (Jake Nutty) is lacking in his performance. A suspicious Chin soon discovers that Jesse is being forced to throw the game or his kidnapped father will be killed.

San Francisco Police Department Inspector Abby Dunn (Julie Benz) works with Five-0 to learn and set up a similar task force in her city. The team investigate the murder of a con-woman and suspect her partner, Hank Weber (Kristoffer Polaha), who claims to be targeted as well. Five-0 work with him to catch the killer, but it is revealed to be a trap to let Hank install a virus on Five-0's tablet table. Hank eventually reveals that he was hired to do this by Gabriel Waincroft. Meanwhile, Adam is kidnapped by Tom Bishop to be escorted to Yakuza leader Goro Shioma (Akira Hirayama). Adam kills his captors and escapes, but is quickly recaptured and sent to Goro, where he learns that his debt has been paid, by Waincroft. Dismayed at the knowledge he killed two people for no reason, he turns himself into the police, with Kono witnessing his confession.

The body of university professor Elliot Thomas is found being fed to pigs and Max determines he was shot. As Jerry is given the task to wait for the pigs to defecate the bullet, Danny and Eric are enlisted to go undercover as a stand-in professor and student respectively to find the killer. Meanwhile, Chin bonds with Inspector Dunn during her stay in Hawaii. Hank Weber is found dead. Kono tells the team that the prosecutors do not believe that Adam killed the two Yakuza members in self-defense and that he is willing to accept a two-year sentence. Also, Nahele (Kekoa Kekumano), a street kid under McGarrett's charge, learns his father has been released from prison and has recovered custody rights. Nahele reveals he does not want to return to his father, because as a child he witnessed his father murder a man.

Ben Nakano, brother of local boxer Luke Nakano (Lewis Ten), is found beaten to death on the day before Luke is to fight opponent Devon Haynes (Harold House Moore) for welterweight champion. The team suspect Juru Katsu (Kelemete Misipeka), a known associate of Gabriel Waincroft, who’s determined to take over the island’s criminal underworld by eliminating several crime bosses. Meanwhile, Adam and Kono spend the former's last day of freedom before he starts his 18-month sentence in Halawa prison.

Flashbacks reveal Kamekona's (Taylor Wily) past as a drug pusher for a local gang, and his first encounter with Chin, ultimately becoming his confidential informant to help arrest the gang. In the present, one of those members is found dead as another, Levi Sosa (Maurice Compte) is released from prison. Chin, Kono and Lou ask Kamekona to go undercover to find out who the killer is. Meanwhile, McGarrett and Danny go to Maui where McGarrett realizes he accidentally signed them up for couples' counseling. Adam receives an unexpected visit from Gabriel Waincroft while he is in prison.

A teenager is shot while on a camping trip, which leads Five-0 to investigate the theft of old unstable Japanese World War II bombs stolen from a hidden bunker. Meanwhile, McGarrett's Aunt Deb (Carol Burnett) comes back to Hawaii after her husband Leonard passes away and reveals she is close to death and has a list to do before she dies. Also, Chin and Inspector Dunn take their romance to the next level.

Lou travels to Chicago to try to get his ex-partner Clay Maxwell (guest star Mykelti Williamson) to confess to killing his wife. Meanwhile while trying to win a bet with McGarrett and Danny, Kono and Chin are held hostage by two gunmen who are trying to bury a body of a fellow police officer.

Launa Cruz is shot, and Five-0 later find a connection between her, another woman named Natelie Jacobs and Jack Cruz, Launa's husband. As Five-0 continue their investigation, they share sides of unlucky experiences during Valentine's Day period. On February 16, on Stakeout day; the team finally gets Danny to tell his Valentine's story that involved Melissa. Following the case, the rest of the Five-0 teams makes McGarrett tell his Valentine's story. Jerry gets his first date; Lou and his wife Renee get a dinner made by Chef and McGarrett is revealed to delete Catherine from his phone contacts. Kono is left watching footage from her wedding with Adam, but still holds a smile. Chin gets a call from Abby, who apologizes for her behavior that ruined their Valentine's Day. Despite her saying she is in San Francisco, she is revealed to be sitting in a car on a mountain top in the outskirts of Oahu.

Five-0 teams up with a man named Neil Palea (Joe Egender) who has autism holds clues to the murder of his friend. They later find out the case is related to an attempted bank robbery with the thieves being hunted and executed by Chin's brother-in-law, Gabriel Waincroft. Chin and Gabriel have a fight, culminating in Chin throwing Gabriel out of a window three stories to the roof of a car. However, Gabriel still manages to escape before Five-0 can arrest him. Meanwhile, Danny and his mother, Clara, (Melanie Griffith) are questioned by the FBI about Matty, but Danny deduces that it was all a ruse to get Danny to admit that the team uses Five-0 resources for illegal purposes. Danny leaves in disgust, and it is revealed that Abby is an FBI agent who is there to arrest Five-0 the next time they cross the line.

Five-0's confidential informant Sang Min (Will Yun Lee) is accused of murdering one of his criminal partners. Sang Min reveals that he was set up when the police arrived. McGarrett seeks the help from his old friend Odell Martin (guest star Michael Imperioli) to help defend Sang Min from the charges because he believes that he's innocent. Elsewhere, Danny travels to New Jersey with his mother Clara to retrieve the money that Danny's brother stole before he was killed and bring it to the FBI.

Five-0 investigates the murders of two hospital surgeons and the patient they were about to operate on. They determine that the patient was the real target as Max shows them X-Rays where a small object was embedded in his skin recently. The vic was actually a mule, carrying a flash drive with NSA intel to Hong Kong for someone else. Meanwhile, Danny returns from New Jersey and informs the rest of the team that the man after them is Robert Coughlin (Ingo Rademacher), Rex Coughlin's brother. Chin and Grover relate Rex's history to Abby: that he tried to bring Chin down on anything and even struck a deal with Gabriel Waincroft to do it. They believe that Robert is after them because of Rex's death. Abby informs Robert that she doesn't think Five-0 is corrupt, but she is told to get something on Five-0 and stay undercover or she'll be fired. At the end of the episode, Abby confesses to Chin that she is a snitch and what kind of a traitor and double agent she is.

Danny's day off with Grace and Charlie starts off bad when Grace is unhappy about being without her phone, but it quickly gets worse when the camaro is stolen. Meanwhile, the team investigates the death of a man found floating in the sea with his friend. Five-0 determines that the men were slaves aboard a fishing vessel. Meanwhile, in the aftermath of her confession, Abby leaves SFPD and McGarrett offers her an extended stay on the Five-0 task force, an invitation that Abby accepts, pleasing Chin.

Lou is forced to go off the grid when his, and his family's, lives are threatened because of an undercover assignment Lou did for the FBI in the past, which has inevitably caught up with him. McGarrett, Kono and Chin launch an investigation into Lou's disappearance and it's revealed to be a hit organized by Lou's former best friend Clay, currently in prison, as revenge on Lou for having him sent to prison for the murder of his wife. A race against the clock begins to save Lou and his family.

The team investigates a stolen $80 million indestructible suit of armor, developed for the U.S. Army as next-gen combat hardware, which was used to later steal a gun used in a murder committed by a Mafia boss. Meanwhile, Jerry's sister, Isabel (Zuleyka Silver), visits Hawaii under the pretense that she's on the island to visit her big brother, but Jerry later finds out that she stole an elephant from the circus that's currently in town and that she needs his help to get it to a proper sanctuary. Also, Kono grows concerned when she found out that Gabriel Waincroft visited Adam in prison and subsequently offered him a job in expanding his new criminal empire. Adam denies accepting the job, but Kono has her doubts.

The team are called up on to spearhead a high-profile manhunt for several dangerous criminals who escaped from their transport via chemical explosion and figure out the mastermind behind the hit. One of the convicts who died at the scene was James Hamasaki and one of the escaped is Adam. Shackled together in pairs, Adam and the other five convicts attempt to flee in the jungle. Chin and Jerry discover that Michelle Shioma, daughter of the late Goro, was really targeting Adam, as James and Adam shared a similar appearance. Steve, Danny, Kono, and Grover search for the convicts using the trail Adam left behind. Meanwhile, Steve discovers that another of the deceased is Nahele’s dad.

Addison Wells (Cassi Thomson), a 22-year-old college student, is kidnapped from the house at which she is babysitting. Five-0 discovers that the abductor hacked into the house's thermostat, baby monitor, laptops, alarm system, and everything else. To crack the case, Steve and Danny recruit their old friend Toast (Martin Starr) who has become a famous multimillionaire for creating the popular phone app "Poopy Penguins". Meanwhile, Kamekona has unveiled his latest enterprise: inter-island booze cruises. With Flippa and Max, the trio set off on the maiden voyage, only to run out of wind, battery power, and food.

Vanessa Diaz is found murdered in her home; Five-0 criminal informant/crime scene cleaner Gerard Hirsch (Willie Garson) is attacked when he discovers the house had a safe room and someone jumps out. Kono is sent to watch Hirsch until his attacker is caught. Five-0 learns that the victim had a daughter named Sara that has an uncle in HPD: Chin. When Chin goes to talk to her, he learns that Sara's aunt was Malia, Chin's deceased wife, and that her father is Gabriel Waincroft. Steve and Danny put two and two together and realize that Michelle Shioma is going after Waincroft through his family by killing his wife and kidnapping his daughter. Meanwhile, Kono meets Hirsch's father Leo (Elliott Gould) who asks her to keep Hirsch on the honest path.

As a meth epidemic sweeps Oahu, Max decides to take the Doctors without Borders opportunity (previously mentioned in I'ike Ke Ao) leaving the ME duties to Dr. Shaw. Meanwhile, Five-0 captures longtime nemesis Gabriel Waincroft, who is badly injured when a drug addict shoots him by accident. As Five-0 attempt to leave, Abby is captured by Michelle Shioma's Yakuza hit squad. Abby later escapes and pleads with her former boss, Robert Coughlin (Ingo Rademacher) to help her track down Shioma's mole in HPD that has been hampering Five-0's efforts to extract Gabriel. Chin and Gabriel separate from the others and make it to an apartment building where Gabriel apologizes for killing Chin's father and asks Chin to take care of Sara. Five-0 gets Gabriel to a hospital, but he goes into cardiac arrest during surgery and dies. Steve, Danny, and the rest of HPD surround Michelle Shioma's house to arrest her, but they discover she took a secret tunnel and disappeared into the wind.

After a chilling meeting with Wo Fat's father in Morocco, Steve returns to battle Hawaii's burgeoning meth epidemic. One of the victims is a friend of Nahele's, so Five-0 returns to the building where they captured Gabriel (in "Pa'a Ka 'ipuka I Ka 'Upena Nananana") to find the source of the meth. Steve and Danny go undercover as drug pilots for Dae Wan (Sung Kang). Midflight, an unknown helicopter attacks the plane and gravely wounds Steve. Danny breaks cover in order to receive instructions on how to land and manages to land the plane on the beach instead of the ocean in order to save Steve. The team learns that Steve ruptured his liver and needs a new one fast. Everyone volunteers, but Danny announces that he and Steve have the same blood type, and offers his liver. Grace, Nahele, Kamekona, Flippa, Max, Jerry, Duke, Pua, Chin, Kono, and Grover wait in the hospital as Steve and Danny undergo the transplant. Later, it is revealed Steve and Danny will make a full recovery. In the end, Steve and Danny are seen arguing about thanks and TV channels.