McGarrett and the team support Danny & Rachel by determining what happened when Grace gets into a life-threating car accident; Tani helps Koa when his rehab patient goes missing.
Adam, Junior, and Tani travel to the bottom of the Pacific Ocean when a murder takes place in a remote underwater lab and the killer may still be on board.
While investigating Flippa's friend's murder, McGarrett and Grover uncover a deadly plan by an extremist group on the island. Also, Grover has an emotional reaction to the case.
While a hurricane lashes Oahu, the Five-0 must house a federal prisoner with a history of escaping. Danny has houseguests during the storm, while an evacuation patrol puts Tani in a dangerous situation.
Tani and Junior go under cover at a gym to track down a deadly batch of steroids; Danny and MacGarrett work as bodyguards for Danny's high maintenance ex-mother-in-law, a famous romance novelist with whom he has never gotten along.
The team investigates the murder of a window washer and finds that the victim is actually a renowned street artist; Grover and Adam look into the murder of a gambler; Junior helps his father when they are reminded of his sisters death.
Five-0 must locate a teen girl who purchased a gun from a junkie, and discover that the firearm, over the course of decades, has affected all of their lives.
Five-0 investigates the murder of a man whose body may hold the key to a 20-year-old case involving a kidnapped girl whose father never stopped looking for her.
While Adam investigates the murder of an undercover FBI agent about to wed the daughter of a Yakuza leader, Noelani is held hostage and forced, along with her mentor Dr. Chu (Page Leong), to operate at gunpoint on a criminal.
McGarrett helps his sister spy on shady neighbors; a ride-share driver and his passenger are murdered; the team teases the youngest members when Junior escorts Tani to a wedding.
Fugitive hacker Aaron Wright (Joey Lawrence) resurfaces to ask Five-0 for help when his co-workers at the NSA, which he has been working for, are all murdered. Also, Junior speaks at the parole hearing of the drunk driver who killed his sister.
Five-0 continues to hunt down escape criminal Aaron Wright before he can sell cyber-weapons to the highest bidder; the lives of McGarrett and the team members are put in danger when a visitor comes to headquarters to make amends.
Tani and Junior must plot an escape for themselves and several civilians after they become trapped inside a deadly tunnel collapse; Eddie the dog sniffs out a bomb that has been planted in McGarrett's home.
A plane that mysteriously crashed into the Pacific in the '80s is found, but with a recently murdered scuba diver inside; McGarrett and Quinn zero in on who planted the bomb in his garage; McGarrett formally asks Quinn to join the team.
When the CIA informs McGarrett that his mother has gone rogue on her undercover mission and may have killed her partner, Steve goes solo to Mexico to bring her home and prove her innocence.
Five-O teams up with a DEA agent to search for a killer whose plane crashed in the jungle with $10 million worth of heroin on board. Also, Grover is thrilled when his niece Siobhan comes to try out for the University of Hawaii basketball team.
It's Thanksgiving, and while Junior and Tani track down the thief who robbed his parents' home, Five-O investigates the murder of a beloved philanthropist and the theft of his ultra-valuable koa tree. Also, Danny moves in with McGarret.
When Adam's girlfriend, Tamiko is kidnapped right in front of him, he shuts out Five-O and breaks all the rules in order to get her back. Also, Five-O investigates three seemingly unconnected homicides.
A tourist helicopter is shot down by a missile; a search for stolen artillery leads to a ghost from McGarret's past -- Wo Fat; Adam makes a bold decision about his future.