It's 1952 in smog-shrouded London. Christmas might be fast approaching, but with her mum away and Uncle Frank busy running the post office, Penny Black is lonelier than ever.

All that changes when Penny discovers a small, fluffy, funny, springy and - most importantly - talking creature in the post office one night, trying to make off with a letter. But Wishyouwas is no thief. He's a Sorter, and he soon introduces Penny to a fascinating secret world hidden in the tunnels underneath the city's streets. Self-appointed guardians of lost mail, the Sorters have dedicated their lives to rescuing letters that have gone astray and making sure they get delivered to their rightful owners.

Penny is determined to protect the Sorters, but how long will she be able to keep them safe with Stanley Scrawl, the sinister Royal Mail Rat Catcher, on the prowl? Can Penny save the Sorters and deliver a joyful Christmas?