Starting in the Easter term after the expulsion of Thekla Von Stift, Jo is stunned to learn that she and the other members of the 'quartet' (Frieda, Simone and Marie) are to be domiciled in the new building with the Middle School. Jo is at first flabbergasted and then dubious about this idea, as it is her last term: in spite of the pretty room she has all to herself, she feels she would prefer to stay in the old Chalet where she has been so happy.
Stacie returns to the Main School 'a different girl', and begins to be accepted by her year. In the process of a visit to Innsbruck to the dentist, Jo and Frieda discover a long-lost relative of Dr. Jem's, with interesting repercussions. On her return, she proceeds to fall foul of the new Matron, Matron Besly, a 'fluffy little idiot' with an overinflated idea of her own importance, who annoys everyone by repeating Matron Webb's mistake and confiscating all books from the dormitories.
Some of the more enterprising Middles forgather on the flat roof one night, to be caught and trapped by Jo; the 'joke' they were planning for Cornelia goes wrong, scares a housemaid into fits, and lands them all into extremely hot water.
Because of Jo's problems with the new matron, including an argument regarding Sunday supervision of the Middles, and an authorised trip to the Hotel to listen to a Tsigane band, the SSM, the Society for the Suppression of Matron, (see A PRINCESS OF THE CHALET SCHOOL) is reformed by Evadne Lannis and Cornelia Flower, but the older prefects recognise the symptoms, and 'Charlie' squashes it before too many pranks are played. Matron Lloyd (Matey) tries to show the new matron where she is going wrong but with no success.
Biddy O'Ryan, now a Middle in the school, creates havoc when she is coerced into telling Irish ghost stories after lights out. The school takes another trip up the Zillerthal; and Cornelia and her cronies start their own 'orchestra' on the quiet, resulting in an end of term 'concert'
However, the most stunning news comes from Marie von Eschenau, although there are hints along those lines in earlier book.
If you want to find out more, click on Spoilers
As the link suggests, it gives more of the story - without, of course
giving everything away!