Steve receives a posthumous letter from his mother that contains a cypher. Danny is attacked by someone willing to kill to steal it. Also, Five-0 must help Lincoln Cole (Lance Gross), an anonymous good Samaritan who's in the crosshairs of some very dangerous people.
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 murder. Also, Tani and Junior work on the nuances of being in a new relationship.
Five-0 investigates when a rancher is murdered after uncovering human skeletons on his property where legend says that Civil War-era gold coins were buried. Also, Quinn's former stepdaughter reaches out for help after her father doesn't come home.
Five-0 investigates a murder 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'sYakuza operation once and for all. Also, Quinn helps Noelani when she suspects that her uncle didn't die of natural causes.
On Valentine's Day, Tani and Noelani are held hostage during a convenience store robbery. Also, Five-0 investigates the murder of a Thai diplomat whose wife confesses to the crime, but Danny suspects she's covering for the real killer.
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 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. Also, Higgins gives Tani some personal advice.
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, Wo Fat. Also, after Adam's ordeal in rescuing Tamiko and reconnecting with the Yakuza, he makes a bold decision about his future with Five-0.
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.
It's Thanksgiving, and while Junior and Tani track down the thief who robbed his parents' home, Five-0 investigates the murder of a beloved philanthropist and the theft of his ultra-valuable koa tree. Also, Danny moves in with McGarrett.
Five-0 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.
The CIA informs McGarrett that his mother has gone rogue on her undercover mission and may have killed her partner. Steve decides to go solo to Mexico in order to bring her home and prove her innocence.
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. Also, Tani and Quinn are stuck with a pair of YouTubers who are brought on a ride-along with Five-0.
Five-0 investigates when a plane that mysteriously crashed into the Pacific in the '80s is found, but with a recently murdered scuba diver inside. Also, as MacGarrett and Quinn zero in on who planted the bomb in his garage.
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