Sergeant Chris Ballard and Lieutenant John Russo are investigating an amusement boat. It goes outside the three mile limit where drinking and gambling takes place.
It's a pleasure boat that attracts waif, strays and juvenile delinquents. The dice are loaded but Ballard and Russo end up investigating an attempted murder.
There is a serial killer in the city strangling bleach blond young women. He is now on the boat but the victim he tried to strangle is not dead.
Unbelievably making out that he is a medic, he is now helping the victim to recover. Waiting for the right time to silence her permanently.
It was odd that given that the Detectives knew one of the passengers was a killer, they would not think if this nervy young man could be the killer. They even left her alone with the victim at one point.
Director Arthur Hiller keeps it tense given it is all rather silly.