Professor Popper (Charlie Drake) develops a shrinking formula with which he plans to solve the world food crisis.
The idea is that vast stocks of food can be sprayed and reduced to a single plane-load, flown to any famine area in the world and then treated with an antidote which restores the load to its original size.
But the eccentric professor and a group of school kids take the formula by accident, ending up trapped in his laboratory.
The plot thickens when a gang of London villains plan to steal the formula and spray it all over the Bank of England, reducing it to the point where it can be packed in a suitcase and sold to the Americans.
Popper hangs on to the formula in a hair-raising chase with a giant tarantula spider and a remarkable ride through London aboard a roller skate.