Billy Bateson is a keen angler, and his friend Gobby a budding scientist like his inventor father. When they find a dead fish upturned in Billy's favourite fishing pond, Billy takes it home for analysis, only for his cat to steal and eat it. The cat collapses and is taken to the vet, who reckons the cat was chemically poisoned.

When Billy sets off on his bike for the pond the next day, he is forced off the road by a massive tanker. At the pond, he and Gobby find many more dead fish and retrieve another specimen. Gobby's father provides an analysis showing chemicals destroyed the oxygen in the water and suffocated the fish.

Gobby sets up water monitoring equipment and cameras to catch any dumpers. The chemical levels continue to rise, but no-one is caught dumping. Investigating geological maps, the boys discover an underground stream flowing into the pond.

The stream's source is traced to an abandoned quarry. From hiding, the boys see the tanker that almost ran Billy over and two men in boiler suits and chemical masks pumping waste into the mine workings.

Although almost caught by the dumpers returning, the boys manage to introduce dye to the stream to prove this is the source of the pollution. A green patch in the pond the next day confirms their theory.

The boys give a statement to the police, who reckon the local Con-Chem plant, where washing powders Breezee and Zap are produced, could be responsible. But it would take hard evidence to implicate it. The boys join a public tour of the factory and Gobby captures video evidence of the tanker driver taking money from a Con-Chem employee. Reviewing the tape, Gobby is disappointed to find the sound and picture is poor, not providing wholly convincing proof.

The tanker makes a dawn delivery the next day, but when it spills some of its load an old Welshman's moped skids off the road. The moped driver alerts the police.

When the tanker arrives at the quarry, Gobby, using a remote speaker system devised by his father, pretends to be the police rounding up the dumpers. But the driver tumbles the ruse and locks Gobby in the tanker's cab. The police and the moped driver have followed the tanker's trail of spilled waste to the quarry and arrive in time to rescue the boys and capture the dumpers.