Arabella Twigg is an ordinary girl living with her extraordinary family. She loves her Gran, but tries to avoid her brother, who is always trying to get her into trouble.

One day, when Arabella looks for some clothes in the cupboard under the stairs, she finds Stuart Whynniard in hiding, who tells her that his father's house is rented out to a group of Englishmen who are planning to blow up the Houses of Parliament.

The lead conspirator is Guido (Guy) Fawkes, who has the dangerous job of lighting the fuses to the gunpowder.