Bud and Sandy discover a bullet-riddled car at the bottom of the bay. Johnny, the barge operator who dumped the car, knows it's evidence of a murder but says nothing to the boys. Johnny's partner-in-crime Dick decides the car should be dynamited. Johnny dives to plant the charge but drops it when the Ricks boys swim into view. He locks the boys in the submerged car then flees. Flipper grabs the dynamite and carries it to Porter who disposes of it far out to sea. Flipper leads Porter to the trapped boys and the two are freed. The police apprehend Johnny and Dick.