RAF jets scramble when radar tracking picks up a UFO but no craft is located. Suspecting faulty equipment, Sergeant Fielding's radar team misses the crash-landing of a football-sized spherical spaceship in Fielding's shed. A smaller silver sphere emerges and bounces into the Fielding home.

Fielding's wife and son Max come home to find food eaten through. Suspecting mice, they lay traps. That night, the silver ball takes the chocolate bait but is able to melt its way out of the trapwire. As the ball emits power, RAF tracking picks up a transmission.

Next morning, yet more food is missing, with curious burn holes in the packaging. Max finds the ball behind a box of biscuits and pockets it. In a big store to buy a rat trap, Max is accosted by Pete, a boy hiding from a shoplifter called Filthy Potter. Potter grabs Pete and takes the 'glitterball' from him. The ball burns a hole in Potter's pocket and zooms off around the shop, eating its stocks. Filthy is chased by security and Max retrieves the glitterball.

At Pete's treehouse, the glitterball dips itself in spilt paint and writes "Help - food". At Max's house they feed it a large pan of custard - re-energised, it is able to communicate via the radio. It explains it is from the planet Stargon and stranded on earth due to engine failure. It must signal home but needs more energy. The glitterball leads the boys to its ship in the shed and is able to magically open the shed's padlock. Potter, watching from the bushes, thinks how useful the little ball could be for thieving.

The boys take the glitterball to an electricity pylon to recharge and send a distress signal but then Potter snatches the glitterball and its ship from them.

An ice cream van helps the boys chase Potter to the store, where he is using the glitterball to open the office safe. The glitterball heats up, burning Potter's fingers, and escapes into the closed shopfloor. The alien mothership approaches earth and drops hundreds of glitterballs into the store. The glitterballs attack Potter and deliver him into the hands of a passing policeman.

The glitterball bids goodbye and returns to the mothership with its compatriots. Max's father, whose radar van has been tracking the glitterball's transmissions, arrives on the scene just too late to witness this UFO sighting.