A Japanese World War II veteran, Yoshio Nagata, escapes from a mental hospital and tries to carry out his wartime orders to blow up a refinery.

A university professor, Dr. Paul Farrar, is using hypnosis to programme students into becoming assassins. McGarrett uses a policewoman as bait for Dr. Farrar, but the plan goes wrong and her life is put in danger.

A psychiatric worker, George Loomis, is trying to drive a fellow Vietnam veteran Ted Frazer crazy in order to cover up the fact that he massacred his own platoon during an action. Loomis kidnaps boys and drugs Frazer to make it look like Frazer is mad and to blame for the tragedy. McGarrett and the team must unravel this web of lies and clear Frazer.

Dr. Alexander Kline, a biological scientist, views his work as evil, and to prove it, he plans to unleash a deadly germ that would kill thousands. Three dead cows lead McGarrett on a desperate chase to track down the toxin, involving a friendly telephone operator and top Washington agents.

Dr. Kline's deadly toxin is lost, he is having second thoughts about his plan, and McGarrett has to find the vial before it disintegrates and poisons the whole island. Already the toxin is doing its deadly work as innocent people come into contact with it.

Five-O investigates the death of Vietnam hero Jack Rigney, who fell from a balcony. His father is a World War II veteran, but his brother Michael is a peacenik, and Danno goes undercover as a draft dodger to help solve the mystery of Jack's death.

A hippie robs an embezzler of the key to a locker containing $250,000. Meanwhile, Danno is trying to contact the daughter of his Berkeley University landlady, who happens to be the hippie's girlfriend…

When McGarrett arrests a group of vigilante nationalists, hidden bombs at the courthouse threaten to destabilise the islands.

The son of a Federal Narcotics officer becomes involved in a heroin refining operation.