Young Tom Long, quarantined away from his sick brother, goes to live for the summer with his Aunt Gwen and Uncle Alan. Though good intentioned, Tom's Aunt and Uncle have little experience in dealing with a growing boy and Tom finds himself dreading the coming months in their care. The apartment, in a big rambling house, is grim and dull and to make matters worth, there's no garden in which to play But something incredible happens to change Tom's summer and his life. One night, as Tom lies in bed, the old grandfather clock in the downstairs hall strikes - thirteen!
Tom investigates this mysterious occurrence and discovers that a magical transformation has taken place. Outside, where during the day there is nothing but a yard with old cars and rubbish, is now a spacious, inviting, fantastic garden. Each night Tom explores the garden and its mysteries. And nothing is more mysterious than Hatty, the young girl Tom meets there. Tom and Hatty form an intense bond, but ultimately, although Tom remains the same age in the garden, Hatty grows older, and more distant from Tom. Tom's desperate attempts to keep his relationship with Hatty cause him to becomes trapped in the past.
After a series of adventures, Tom returns to the present. But there is one more discovery left to make - the identity of the old woman who lives in the apartment above his Uncle and Aunt.