Hardie's attempt to help a stranded man goes awry when he realizes that man has a $25,000 bounty on him.
Hired to test a new electronic fish finder, Mike Nelson cruises to a small Mexican fishing village where he finds a strange phenomena - every fish in the vicinity seems to have disappeared. Mike uses the latest scientific "miracles" to pierce the mystery of the disappearing fish and save a fisherman's life - and the life of the entire village.
Gathering museum specimens in the coastal waters off lower California, Mike Nelson learns of a derelict mine which has drifted into the harbor of a nearby town. The town's principal industry - a nitrate factory - will blow up and destroy the city if the mine explodes. Mike works with local fishermen to locate the drifting "killer" before it costs hundreds of lives.