It is 1957, and Tom Long's brother, Pete, has the measles. Although he has no symptoms, Tom is dispatched to stay with his Aunt Gwen and Uncle Alan who live in the second floor apartment of a converted Victorian country house. There's no garden, just a small parking lot, because new houses have been built on the old grounds, which were sold to a developer. The third floor of the building is occupied by old Mrs Bartholomew who also acts as the landlady. Because he was around Peter when his brother was getting the measles, he is kept away from everyone in case he is contagious. He starts to go a little bit stir crazy and he is feeling very lonely.
It is almost impossible for Tom to sleep because he is sitting around inside all day and not getting any exercises at all. One night, he hears the large grandfather clock in the building's hallway strike thirteen, which is curious enough to make him get out of bed, go downstairs and investigate. He finds that the back door is open, but instead of leading our onto the parking lot it now opens onto a large walled garden that is bathed in sunlight. He finds a sundial in the garden, and when Tom looks around he realizes that he is in the Victorian era and that this must be what the house and garden looked like before it was converted into houses and apartments.
Every night the clock strikes thirteen, and every night Tom goes out into the garden. He meets a little girl who seems just as lonely as he is; her name is Hatty and they become best friends and much-needed playmates. Hatty's family live in the house with her but they cannot see Tom, although he can see them.
Although Tom keeps his new friendship and his slipping into the Victorian era a secret from his aunt and uncle, he writes to Peter frequently and tells him about Hatty, and the games they play. Peter is well enough now for Tom to go home again, but he doesn't want to, because he is enjoying his friendship with Hatty, and his ability to travel backwards in time when the clock strikes thirteen. The strange thing is, though, that even though Tom is visiting the garden on consecutive nights, he is actually slipping back into the Victorian era at various different random moments in Hatty's life. She is growing up far more quickly than he is and is overtaking him rapidly. She is soon an adult, and can't wait to tell Tom about the young man whom she has her eye on. They have started dating, and his name is Barty.
The night before he is scheduled to go home again, Tom goes downstairs at the thirteenth strike of the clock, but the garden has disappeared. He runs around the perimeter of the house to try to find it again but ends up running into a collection of garbage cans from the present day. Devastated, he calls Hatty's name a few times but gets no answer. Uncle Alan comes downstairs to find Tom sleepwalking; he has woken up most of the residents of the building and the next day is ushered upstairs to Mrs Bartholomew's apartment so that he can apologize to her for being so noisy. Mrs Bartholomew does not tell Tom off; instead, she reveals that she was Hatty, and she heard him calling out her name just before he ran into the garbage cans. She reassures Tom that he did not imagine his adventures in the garden, nor did he dream them. He was slipping from the present into Hatty;s real-life past, going back into the time that old Mrs Bartholomew happened to be dreaming about each night. That final night, she hadn't been dreaming about being in the garden; she had been dreaming about her wedding day, when she had married Barty.
When Tom leaves, he gives Mrs Bartholomew a huge hug, rather as if she was a little girl and not an elderly lady. Aunt Gwen finds this so unusual that she mentions it to Tom's mother when they take him home.
