Tom is temporally living with relatives, in a converted house and a clock that strikes thirteen. At midnight, he sees a large garden and two 'ghosts'. Next day it has gone only to appear at midnight. Tom can explore and no one can see him.
Tom discovers he can walk through solid objects and Hatty can see him. He soon learns months pass every time he visits. Next night Hatty is now fifteen so Tom goads her to climb a tree but is helpless when she falls heavily to the ground.
Hetty has recovered and agrees to place a watch, in a secret hiding place, so Tom can find it in the future. On his last night, Hetty is now seventeen can no longer see Tom. Mrs. Bartholomew introduces herself to Tom and says she is Hetty.