Based on the book by Terry Pratchett, the adventure begins for thirteen-year-old Johnny Maxwell and his four friends when they come to the aid of eccentric homeless woman, Mrs Tachyon, and are left minding her trolley full of black bags while she is hospitalised. When the children later visit her in hospital, Mrs Tachyon lets them into the secret that her black bags-- or 'Bags of Time-- are special and can be used to create a gateway to the past. The five youngsters end up during the Blitz where fun and games turn serious when they quickly realise the prejudices of the era. But when they return to their own time, the gang discover they have somehow changed history as a result Johnny's grandmother died in a bombing as a teenage girl meaning Johnny's mother and, in turn, himself were never born. The children are now in a race to fix the path in order to save the future but can they succeed before the bombs start falling...?