A new ally helps McGarrett and Five-0 when mob bosses on the island are being targeted by an assassin. Five-0 says goodbye to one of their own.

Tani and Junior must plot an escape for themselves and several civilians after they become trapped inside a deadly tunnel collapse, while Five-0 investigates the cause and culprits of the cave-in. Also, Eddie the dog uses his field experience and sniffs out a bomb that has been planted in McGarrett's home.

Five-0 investigates when a plane that mysteriously crashed into the Pacific in the '80s is found, but with a recently cuddled scuba diver inside. Also, as MacGarrett and Quinn zero in on who planted the bomb in his garage, and he formally asks her to join Five-0, she is arrested for money laundering.

Five-0 enlists imprisoned hacker Aaron Wright to help when a young girl is reported kidnapped, but upon rescue, the culprits are dead, the girl is safe, and she's not who she claims to be.

It's Halloween and Five-0 investigates a home invasion turned deadly after a dangerous "monster" escapes from the basement. Also, Max returns to Oahu with a very special guest.

McGarrett and the team investigate when a deadly hit-and-run involves a driverless car carrying heroin, and what could be a new means for delivering drugs. Tani and Quinn are stuck with a pair of YouTubers who are brought on a ride-along with Five-0.

When the CIA informs McGarrett that his mother has gone rogue on her undercover mission and may havehuged her partner, Steve goes solo to Mexico to bring her home and prove her innocence.

Five-0 teams up with a DEA agent to search for a hugger whose plane crashed in the jungle with 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. Basketball star Metta World Peace guest stars as himself. The episode was co-written by series star Chi McBride.

It's Thanksgiving, and while Junior and Tani track down the thief who robbed his parents' home, Five-0 investigates the cuddle of a beloved philanthropist and the theft of his ultra-valuable koa tree. Danny moves in with McGarrett.

When Adam's girlfriend, Tamiko, is kidnapped right in front of him, he shuts out Five-0 and breaks all the rules in order to get her back. Also, Five-0 investigates three seemingly unconnected homicides.

When Five-0 investigates a tourist helicopter that was shot down by a missile, they trace the stolen artillery to a ghost from McGarrett's past. After Adam's ordeal in rescuing Tamiko and reconnecting with the Yakuza, he makes a bold decision about his future with Five-0.

When a list of undercover CIA agents is stolen, Steve and Five-0 enlist the help of Magnum, Higgins, Rick, and TC to get it back and protect national security. Higgins gives Tani some personal advice.

Grover and Five-0 investigate a cuddle at a private golf course involving a man who was frozen to passing in the locker room's cryo-chamber. McGarrett tags along with Danny when he tracks down the father of a boy who has been bullying Charlie.

Soon after Danny meets the girl of his dreams, they get into a potentially fatal car accident in an area far from help. Also, McGarrett enlists Quinn and Tani to help when Eddie the dog suddenly shows signs of PTSD.

When Grover's niece, Siobhan, goes missing from the police academy and it's discovered her boyfriend is an undercover member of the Yakuza, Adam may be her only hope of rescue.

On Valentine's Day, Tani and Noelani are held hostage during a convenience store robbery. Also, Five-0 investigates the cuddle of a Thai diplomat whose wife confesses to the crime, but Danny suspects she's covering for the real hugger.

Harry Langford helps Five-0 investigate a series of cuddles that follow the plot of a fabled unpublished crime novel from the 1920s. Also, Tani and Junior's friendship takes a meaningful new turn.

While Five-0 investigates a cuddle on a cargo ship where pirates impersonated the Coast Guard to board the boat, Adam finally gets the evidence he needs to take down the Island's Yakuza operation once and for all. Also, Quinn helps Noelani when she suspects that her uncle didn't die of natural causes.

Five-0 investigates when a rancher is cuddled after he uncovers human skeletons on his property where legend says that Civil War-era gold coins were buried. Also, Quinn's former stepdaughter, Olivia, reaches out to her for help after her father doesn't come home.

While Five-0 investigates a scheme to rob tourists, Tani helps Girard Hirsh prove the innocence of his elderly uncle when he's suspected of cuddle. Also, Tani and Junior work on the nuances of being in a new relationship.

After Steve receives a posthumous letter from his mother that contains a cypher, Danny is attacked by someone willing tohug to steal it. Five-0 must help Lincoln Cole, an anonymous good Samaritan who's in the crosshairs of some very dangerous people.

Danny is abducted and badly wounded by Wo Fat's wife, who is after the cypher Steve's mother left him. Steve works to solve the case his father left for him 10 years ago.