Friends of the real estate developer Mingo (Jay Robinson), including his associates Wallis (Jason Evers) and Akamai (Ray Danton), gather for a birthday party at the house where the three of them live. After Mingo opens one of his birthday presents with a card attached with a message reading "filthy scum," he collapses, choking, and dies.
Mingo's body is taken away in an ambulance. McGarrett wonders who stands to profit from Mingo's death, suspecting Mingo's two partners. Five-O gets the assembled partygoers to recreate the scene just before Mingo collapsed, but the box Mingo was opening has disappeared, and no one saw what was in the box. Doc (Al Eben) says that something hit Mingo's central nervous system. After Doc performs an autopsy, he says there were unusual hexagonal alkaloid particles present in Mingo's blood. Doc suggests that Mingo was poisoned, but there are no obvious signs as to how this was done.
Later, as Akamai frolics on the beach with a couple of bimbos, he shows Wallis a threatening typed message that he also received under his bedroom door.
The investigation continues. Akamai is not concerned about the threat on his life and goes fishing with Manoa (William Valentine), their butler. Kono is fascinated by dead fish floating in a tank at the developers' pad. Wallis shows Danno a videotape machine in a bedroom which he says was "a sales tool."
Akamai almost drowns when he is skin diving, but is saved by Manoa. As a sign of gratitude, Akamai gives Manoa his expensive watch.
McGarrett is perplexed by the case, unable to figure out the motive for Mingo's murder. He tells the Five-O team to check out the servants and to use a computer to check the partygoers, as well as investigate complaints regarding the trio's real estate activities.
Akamai is found dead by his servants.
Danno arrives to take charge of the ongoing investigation. Wallis wants to leave the islands, becoming more and more paranoid, but Danno says Wallis is not going anywhere since he is "suspect number one" and "target number one." Kono notices that the pillow under Akamai's head was wet with sea water.
Doc does more testing and determines that a deadly poison like curare which paralyzes the nervous system was used to kill Mingo and Akamai. As well, the note which Mingo received was also damp with seawater.
Danno and Kono visit Sea Life Park, where Dr. Pell (John Hunt) explains a shellfish called Cloth of Gold can poison someone with minute needles which leave no marks.
Danno calls the police guarding the developers' house and is told that Wallis is locked in his room.
Manoa brings Wallis his lunch and hands Wallis a Cloth of Gold shell as a gift. As Wallis starts to feel the effects of the poison, Manoa tells Wallis he is taking revenge for his daughter, who the developers drugged, filmed participating in pornographic activities, and turned into a hooker who died of infectious hepatitis.
Danno and Kono arrive at the developers' house, but it is too late. Kono notices that the Cloth of Gold shell is gone from the aquarium. Manoa attempts to flee to the big island on a yacht, but he is trailed by Danno in a helicopter and McGarrett and Kono on a police boat.
McGarrett and Kono eventually catch up to Manoa, and McGarrett boards the yacht. But Manoa has one more Cloth of Gold shell which he touches to his throat, committing suicide. Manoa falls off the yacht into the water.