It is late August 1939: Britain is on the brink of war, and preparations are under way to evacuate London's children to the countryside. When twelve-year-old Tilly and her best friend Rosy find out that they will not be able to take their beloved dog and cat with them - and that, even worse, their pets will, along with countless other animals, be taken to the vet to be put down - they decide to take action. The two girls come up with the idea of hiding them in a derelict hut in the woods and, when other children find out and start bringing their rabbits, guinea pigs and hamsters, their secret den turns into an emergency zoo.