The O'Sullivan Twins at St Clare's recap

Pat and Isabel return happily to St Clare's with their cousin Alison, whose vanity and inexperience make school life difficult. Attention soon centres on Margery Fenworthy, a hostile new pupil whose sporting brilliance cannot overcome the form's suspicion of her. Erica exploits that distrust by damaging Pat's possessions and making Margery appear responsible. Margery nevertheless risks her life to rescue Erica from a sick-bay fire and suffers a broken leg. Erica confesses, leaves St Clare's, and Margery is cleared, befriended, and reconciled with her father. When an accident threatens painter Max Oriell's livelihood, Margery proposes that Lucy attempt an early scholarship so she can remain at school. Lucy begins intensive preparation while Margery rises to the top of the form. In the final conflict, Janet's prank aggravates an exhausted Mam'zelle's illness, but Janet confesses and receives forgiveness. The term ends with Alison reconciled with the twins, Margery recovering, and Lucy's scholarship result still undecided.

Previously: Pat and Isabel O'Sullivan began their time at St Clare's with a poor attitude but changed during their first term. They came to value the school and became settled among the first-form pupils. Their established circle includes Janet Robins, Doris Elward, Kathleen Gregory, Sheila Naylor, and Hilary Wentworth, while Miss Roberts leads their form and Miss Theobald is the headmistress. Mam'zelle teaches French, and Belinda Towers has authority in school games.

Pat and Isabel return to St Clare's with their cousin Alison, who dislikes practical duties and depends heavily on them. New pupils Margery Fenworthy and Lucy Oriell make sharply different impressions: Lucy is clever, artistic, and friendly, while Margery is hostile in class but outstanding at games. Alison's poor effort at lacrosse and mishandling of a senior's fire show how much she must learn. Meanwhile, Tessie arranges a forbidden midnight birthday feast. Erica, excluded from it, discovers the plan and secretly directs Mam'zelle to the music room. Miss Theobald removes the eight partygoers' town privileges for two weeks. Gladys then supplies a clue that Erica knew about the frying pan, and Erica betrays that knowledge when Pat and Tessie confront her. Although she refuses to confess, both the first and second forms identify her as the saboteur and ostracise her. Erica responds by vowing revenge on Pat and Isabel.

Margery briefly responds when Pat, Isabel, and Lucy persuade the first form to treat her more kindly, but her defiance toward teachers soon renews the girls' hostility. Her exceptional lacrosse skill earns her a place against Oakdene. After overhearing Pat urge the school not to cheer her, Margery scores the goals that secure a 4-2 victory but receives no recognition from her schoolmates. Erica then secretly tears apart Pat's partly knitted jumper. Because Margery had threatened revenge over the match, the pupils accuse her despite her denial that she would act covertly. At half-term, former Redroofs pupil Pamela Holding tells Pat, Isabel, and Alison that Margery has been expelled repeatedly for insubordination and refusing to work. Pamela also says the hidden sabotage does not match Margery's usual open behaviour. The cousins promise their mother not to repeat this private history, but the damaged jumper remains unexplained and they still suspect Margery.

Erica ruins Pat's nature-study materials and arranges the scene to implicate Margery. Alison then breaks her promise by publicly revealing Margery's expulsions, leaving Margery condemned by the other pupils. Before the girls can take the matter to Miss Theobald, Margery discovers a fire in the sick bay. Staff mistakenly believe Erica has escaped, but Margery sees that she remains trapped. Margery climbs to the upper window, lowers Erica on a rope made from sheets, and falls after completing the rescue. She suffers a broken leg and a minor head injury. Overcome because Margery saved the person who framed her, Erica tells Lucy that she committed both attacks on Pat. Miss Theobald sends Erica home for the rest of term and arranges her move to another school. Margery is cleared, and the girls apologise. She explains that her mother's death, rejection by her stepmother, and estrangement from her father shaped her earlier behaviour. Pat, Isabel, and Lucy contact him about her courage, leading to a reconciliation and permission for Margery to remain at St Clare's.

While Margery recovers and begins working seriously, Lucy learns that a car accident has left Max Oriell unable to use his right hand normally. The loss threatens his portrait work and means Lucy may have to leave school. Margery proposes that Lucy take the third-form scholarship examination two years early. The teachers judge that she has a real chance and arrange intensive coaching, while warning that the plan must not destroy her later opportunity in art. Margery also reveals strong academic ability and ties Lucy for first place in the form. A final dispute develops when an unusually irritable Mam'zelle punishes Janet severely. Janet retaliates with a false ink blot and insects hidden in the teacher's spectacle case. Mam'zelle, already ill from exhaustion after nursing her sister, believes she is hallucinating and collapses under the strain. Janet confesses both tricks, brings flowers from the form, and is forgiven. By Easter, Mam'zelle has recovered, Alison is reconciled with the twins, and Erica has departed. Margery's leg is healing and her father again supports her. Lucy remains at St Clare's to prepare for the scholarship, whose result is still open.

Erica has admitted sabotaging Pat's work and framing Margery, and she leaves St Clare's to attend another school. Margery is cleared by the confession and transformed in the eyes of the pupils after rescuing Erica from the sick-bay fire. Still using crutches as her broken leg heals, she is working hard, plans to become a games mistress, and has regained her father's support. She will spend part of Easter with Lucy and later join her father for a holiday. Lucy remains at St Clare's under a special programme for the third-form scholarship examination. Winning it next term may allow her to stay despite the financial consequences of Max Oriell's injured hand. Mam'zelle recovers after Janet admits the ink-blot and insect tricks, and teacher and form forgive one another. Pat and Isabel leave for Easter with Alison, whose relationship with them has improved despite her harmful disclosure of Margery's past. Lucy's scholarship is the principal unresolved question.