Third in the Abbey Series, this continues the story of Joan and Joy, the original 'Abbey Girls' and introduces Jen, the third of the main characters. The school is forced to close because of an outbreak of diphtheria, and reopens at 'The Hall' which Joy has recently inherited, and in the grounds of which stand the Abbey. A brother and sister who are left to run wild in the neighbourhood find a series of underground passages. This leads to the discovery of treasure, and of an underground crypt, an early church unknown since the dissolution of the monasteries.