The ghost is Sally
. She is in the hospital, seven years in the future, because Julian Addiman
, now her boyfriend, threw her out of his speeding car when she broke up with him.
The sister "Sally" who comes to see the patient in the hospital is Imogen -- the nurse got her name wrong. Sally did not recognize her because Imogen's musical training has made her unhappy and worried. They are flatmates.
Monigan is a real, ancient, and terrible goddess, whose presence Cart instinctively sensed when she invented the Worship of Monigan.
Sally and Julian Addiman secretly swore fealty to Monigan; Sally's accident is the result of Monigan's attempt to claim her life. The "ghost" is Sally from the future, attempting to warn her sisters and herself that in seven years Monigan will claim a life.
In order to save Sally, whom they suspect is one of them, the sisters and their friends make sacrifice to Monigan.
- Cart offers Oliver, but he is rejected as imperfect.
- Fenella gives up the part of her brain that older girls use to do A-levels; in the future, she is indeed expecting to fail her exams, but is unconcerned, since she is pursuing a career as an opera singer.
- Audrey Chambers offers a bunch of wildflowers which she and Ned Jenkins picked together; in later years, she falls obsessively in love with Ned, who is quite indifferent to her.
- Will Howard sacrifices his tie-pin, which has a Union Jack on it; his family emigrates to Canada, much against his will (although in later years he is very happy there).
- Ned Jenkins offers a picture of Sally, but Imogen reclaims it; Ned and Sally are romantically united at the end of the book.
- Julian Addiman, who has already sworn himself to Monigan, makes no sacrifice, and his life is forfeit; after throwing Sally out of the car, he crashes and is killed.
- Sally, too, has already sworn herself to Monigan and makes no sacrifice; it is her life, of course, which is saved by the others' efforts.
- It is Imogen's sacrifice of her musical career, with all the fame and wealth that imagination can supply, which ultimately appeases Monigan; in the future, learning that her sacrifice was efficacious, Imogen gives up music with delighted relief.