Two men out boar hunting discovered a disoriented woman in the forest looking for help. At the hospital Danno and McGarrett were called in to question the young woman and investigate her possible abduction. An encounter with the woman's mother-in-law placed the young woman under suspicion as a possible suspect.
Chin and Kono discovered evidence that the young woman and her new husband had been gassed with acetone. They also discover the husband's dead body. After a discussion with the medical examiner they learned that the M.O. for the case matched that of unsolved murders that took place a year before in French Polynesia. A search of photos taken by a private investigator that had been tailing the young couple revealed the image of a possible suspect.
McGarrett and Danno searched the forest where the young woman was originally found and discovered the place the killer was going to use to murder the young newlywed. A bottle of morphine found at the scene led the Five-O to track cruise ships, and they discovered that the killer had already murdered other couples on the other islands.
The cruise ship the serial killer was on was held in port by the police while Danno and McGarrett searched the ship. The serial killer managed to slip away, but a briefcase in the killer's room helped the Five-O figure out the next targeted couple. Arriving too late to stop the abduction Danno and McGarrett engaged the serial killer in a high speed chase ending at a dead end mountain road. McGarrett shot the serial killer who then jumped to his death.
Chin looked up an old case number that McGarrett's father had been investigating. He told McGarrett that the file was missing or stolen, but he managed to discover a few things. The case number was linked to McGarrett's mother, and it was for a murder investigation, not a car accident as McGarrett had believed.