Dr. Quest has constructed, and is testing, the ""Underwater Prober"", a cross between a submarine and a tank. The Navy is interested ... and so are some mercenaries.
Dr. Quest and Hadji leave for the mainland, to meet a scientific delegation from Calcutta. While Race and Jonny work to fix part of the machine, they are overpowered and the machine is hijacked. They manage to escape, briefly. Then Hadji and Dr. Quest are captured when they come to the rescue.
Hadji saves the day and a guard gets ""stoned"", but the team isn't out of danger. Fortunately, the Underwater Prober proves up to the challenge. The pirates try one last gambit, and Race turns the tables on them -- they meet a fitting end indeed. Confident the machine is ready, Dr. Quest turns it over to the Navy.