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.

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."

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.

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.

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.

When the bodies of two serial killers are found on Five-0 property with chess pieces in their mouths, McGarrett and the team hunt for a vigilante as fear grows that tourists aren't safe in Hawaii.

McGarrett puts his life in lethal danger when Five-O puts a risky plan to rescue a group of young girls from a sex trafficking ring in motion, Kono decides to make a life- altering choice because of the unique strain caused by the case.