A college professor is murdered, but who is the killer: his boss, his teaching assistant, or one of his students? Meanwhile, Kono is assigned to protect an annoying witness released from federal prison.

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 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.

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.

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 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.

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.

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.

On Halloween, Five-0 investigates the murder of a medium with a checkered past who was apparently scared to death by a poltergeist. Also, Kono and Adam are kidnapped by what appears to be a death cult.

A conspiracy theorist with compelling information that JFK's own cabinet planned his assassination is murdered. Jerry tells the story of the Honolulu Conference on November 20. 1963.

Five-0 uncovers the possibility that a Nazi war criminal may be hiding in an old leprosy colony.

McGarrett and Kono suspect that an abused young girl may be the victim of a sex trafficking ring on the Island. Also, Chin and Grover investigate a murder at a sober living facility.

McGarrett and his team go on the trail of an escaped prisoner convicted of arson.

Five-0 investigates a series of crimes based on legendary island folklore.

When Danny is shot while quarantined in the hospital with McGarrett, Tani and Junior, he's left unconscious and clinging to life, imagining a future for everyone in Five-0.

Five 0 investigate the murder of the headmaster of a private school sending Tani and Junior undercover as prospective parents. Meanwhile Adam is forced to produce a huge amount of hidden money to save his life.

Young Jerry witnessed a murder while at camp in 1982. He's telling this story to his friends while going back to the camp in the present day. He wants to search for the body before developers level the property. A woman watching a news story about a murder realizes that her daughter drew a picture of the crime scene three days earlier. Jerry's most likely site for the body is on the property line of Bo Bradley, who Jerry thinks is the Axe Man. McGarrett and Junior interview the girl. Katie. She says her friend Molly drew the picture. Noelani shows up to help hunt for the body with ground-penetrating radar. in 1982, Jerry breaks into Bradley's house and ran away screaming after Bradley finds him. Noelani finds a bone, but it's canine. Jerry keeps digging because he thinks there's a body underneath, while everyone else leaves as it starts pouring. Three of Katie's pictures were drawn by someone else. Katie's parents see this girl, Molly, on the monitor but she escapes. Everyone's phones and keys are missing at the camp. Tani finds a woman that may have been attacked by the murderer. McGarrett thinks the drawings may be three different redheads. Jerry found human remains, but he's being followed by someone. Gordie knocks on the door and spooks everyone. He took the keys and phones. He goes to get them, but his throat gets slashed. It's the ax man, who set the cabin on fire. They have to run out, but Bradley shoots the ax-man, who is the kids' old camp counselor. The serial killer is Molly's father, who went nuts when her mother divorced him. Jerry goes back to thank Bradley and apologizes to him. Molly's mother comes to get her. Captain Keo said the gun in Adam's house was the one that killed Noriko.