Smiler's grandad turns an old fort into a Laughter Club for children. He rents out part of the building to the mysterious Dr Maudlin. Smiler and Coriander, the only other member of the club, discover that the Doctor is meddling with genetics in an attempt to eradicate head lice. She has a prisoner - a Neanderthal boy brought to the fort in a terrible electric storm and now trapped out of time, together with Neanderthal lice and the terrible metal-eating primeval slime! She must be prevented from wiping out a species. Another storm returns the boy to his proper time and destroys Dr Maudlin's work - but unbeknownst to anyone, the slime has escaped!