A murder mystery sends Five-0 in search of pirate treasure. Meanwhile, Gabriel Waincroft holds Kono and Adam hostage after their wedding and demands the money they were using to pay off the Yakuza.

Five-0 needs the help of an imprisoned bomber to find a copycat on the loose, but is he somehow related to the crime? Meanwhile, Kono and Adam discover that they're being watched after Adam recovers from the shooting.

While the team tries to track down the owner of a gun found in the ocean that was responsible for two deaths, McGarrett and Catherine reach a crossroads in their relationship.

Five-0 investigates when a wealthy recluse is found dead in a remote field with nearly one million dollars in counterfeit bills. Also, Adam is kidnapped by the Yakuza and forced to torture one of Gabriel's men. Jerry Rice guest stars.

Five-0 interrogates members of a stunt biker gang to locate a killer who escaped by jumping his motorcycle from one rooftop to another. Meanwhile, the riders prepare to race in a Tough Mudder competition.

Five-0 is in search of a serial killer who may be a modern-day Dr. Frankenstein, while also dealing with a hostage situation after a blood bank robbery. Meanwhile, Danny is frustrated after Grace goes to a Halloween party behind his back.

While snorkeling off a deserted island McGarrett and his new girlfriend stumble onto the hideout of a wanted gangster while Chin, Kono and Grover must track down a gambler who is forcing a high school quarterback to throw a playoff game.

Five-0 works with a conman to find a killer after his partner is murdered. A San Francisco police inspector helps Chin Ho investigate the murder of Chinese arms dealers. Adam receives some unsettling news after meeting with the Yakuza boss.

After a college professor is murdered, Danny goes undercover as his replacement and Eric poses a college student. McGarrett gives advice to a young man after his dad returns from jail and wants to get back in his life.

The brother of a local boxer is killed the night before his championship bout, and Five-0 connects it to Gabriel Waincroft, who has an even bigger target in mind. Meanwhile, Adam takes a plea and prepares to serve his sentence.

Chin Ho asks Kamekona to go undercover to help solve the murder of a former drug-dealing associate from his past, while Steve and Danny go to a couples retreat to work on their relationship..

Five-0 uncovers a Japanese World War II spy mystery after a young boy is shot in a long-forgotten bunker. Meanwhile, Aunt Deb returns to Hawaii to visit the McGarretts so she can help finish her recently-deceased husband's "bucket list."

Lou takes extreme measures to expose the truth about a friend's death, while Danny, Steve, Kono, and Chin compete in a friendly wager that turns deadly.

The team unravels a plot to murder a pair of unfaithful spouses, while each member also recounts their disastrous Valentine's Day mishaps.

An autistic man helps Five-0 uncover the details of a daring bank heist, which ultimately leads to another showdown with Gabriel Waincroft. Meanwhile, Danny learns that the Feds want to talk to his mother about his brother's death.

When Five-0's sketchy confidential informant is accused of murder, McGarrett asks his old friend Odell Martin to defend him. Michael Imperioli returns and Ziggy Marley guest stars.

The team investigates a bizarre killing at a hospital. Just as a team was going to operate on a man who was brought in, someone posing as doctor killed everyone including the patient. Max tells Steve that the killer also cut the patient. Steve thinks the man had something in him. They locate the doctor who did the procedure who says he placed a flash-drive in the man. The doctor says another guy paid him to do that. They learn he's computer programmer, who works for a company that works for the NSA. His superiors inform Steve that they fear there may have been a breach and the programmer is the one who did it. Steve tries to question him but he escapes. They learn the killer is the programmer's girlfriend and that she's a Russian spy. They find them but she starts shooting and gets away. Steve talks to the programmer who says he thought he was doing something noble but it turns out he was duped by the girl. Steve is then informed that among the info he downloaded has info on someone he knows who is on a undercover assignment. The programmer says the info is safe because he encrypted it. But that's when everyone passes out, when they wake up they find the programmer gone. Evidently she took him to open the file. So they have to find him before he does. Steve also tells everyone who is behind the investigation on them.

Five-0 tries to shut down an offshore fishing boat that is using slave labor. Meanwhile, Danny tracks down a pair of violent criminals who stole his car, and inspector Dunn comes clean about her secret mission.

Grover is forced to take himself and his family off the grid without notice when he discovers that an enemy from his FBI undercover days has tracked him down.

The team discovers that the prototype for a high-tech body armor suit has been stolen, and Jerry finds out that his sister is involved with the release of an elephant that is still on the loose.

Five-0 goes on a manhunt when a chemical spill permits six dangerous inmates to escape, including Kono's husband, Adam.

Five-0 must save a kidnapped college student from a dangerous vigilante. Also Max, Kamekona and Flippa try to survive after being shipwrecked.

While investigating the murder of a woman connected to the drug trade, Five-0 discovers that the victim's daughter has a close personal connection to both Chin Ho and Gabriel Waincroft.

Five-0 captures a seriously wounded Gabriel Waincroft and prepare to bring him in, but Michelle Shioma wants him for herself to avenge the death of her father, and her men will stop at nothing to get him for themselves.

In order to stop a meth epidemic on the Island, McGarrett and Danny go undercover as pilots, but when McGarrett is seriously wounded, Danny is forced to break their cover in order to save his partner's life.