Arabella Twigg is a fairly ordinary girl living with an extraordinary family. She loves her Gran, whilst trying to avoid her brother, Tom, who is always trying to get her into trouble. Her life is quite boring until strange things begin to happen.

Arabella folds her school uniform and places it on the chair one evening, then puts on her dressing-gown and returns to the bathroom. The room is full of steam and as the steam clears, suddenly she is standing on the side of a Roman Bath.

There she meets two young girls from Pompeii who tell her all about the city just before Vesuvius erupts.