The Game is a fantasy novella by award-winning author Diana Wynne Jones. The protagonist, Hayley Foss, is a young orphan girl who lives on the outskirts of London with her grandparents. Her grandmother is very strict and formal. Hayley has no memory of her parents or their deaths and her only link to them is an old wedding photograph her grandmother gave her. Hayley lives a very sheltered life and is homeschooled. She is not allowed to play with other children her own age. She cannot leave the property unless she is escorting one of the maids to the nearby town to make purchases.

During one of these excursions to town, Hayley meets two male musicians who are twins that she nicknames Flute and Fiddle. Hayley mistakes the term "musician" for "magician" and talks to each man briefly about magic. Later, Flute appears in her backyard and takes her on her first journey to the "mythosphere."

When Hayley's grandmother discovers that she is talking to the musicians and that she has traveled to the mythosphere, she accuses Hayley of "romancing" and attempting to destroy her grandfather's work. She arranges for Hayley to travel to Scotland to live with one of her aunts. Hayley is escorted by her older, cousin, Mercer, and is taken to the family guesthouse in Ireland where she meets several of her aunts and many of her young cousins. The cousins play a game of sorts that allows them to travel along strands of fables and fairytales into the mystical world of the mythosphere. Hayley is continuously warned that Uncle Jolyon cannot find out that they are playing this game or he will be very angry. Playing the game leads Hayley to the realization that her family members are actually gods and demigods.

Tollie eventually tells Uncle Mercer about the game. Mercer tells Jolyon and he comes to Ireland to punish the children for disobeying. Hayley is informed that a prophecy says that she is the only person who can stop Jolyon's iron-handed ruling of the other family members. Jolyon chases the children through the mythosphere where Hayley discovers her parents have been imprisoned by Jolyon for marrying. She helps them escape and they are reunited as a family.

With the help of her parents, Flute and Fiddle, a powerful Russian witch (who was once her grandmother's maid), and several of the cousins, Hayley stops Jolyon by turning him into the planet Jupiter and pinning him to the sky.

Once Jolyon is out of the picture, the group starts making plans. Merope and Cyrus consider going back to Greece where Cyrus was formerly King. Troy wants to go back in time and redesign the city of Troy. Harmony will continue to freely travel through the mythosphere. Aunt Aster and the giant Highlander walk down the main street in view of all of the neighbors and Troy tells Hayley that they can all do whatever they want from now on.