The first installment in the Malory Towers series follows 12-year-old Darrell Rivers as she begins her first term at Malory Towers, a girls' boarding school perched on a cliff in Cornwall. Built of gray stone around a central courtyard called the Court, the school has four rounded towers, each housing a separate boarding house. Darrell is assigned to North Tower, under housemistress Miss Potts.
On the train, Darrell meets her classmates. Alicia Johns is a sharp-tongued, lively girl who takes charge of Darrell immediately. Gwendoline Lacey, a spoiled only child previously educated by a governess, clings tearfully to her mother on the platform and must be pried away by Miss Potts. Sally Hope, the third new girl, arrives alone. Darrell likes Alicia and hopes to become her friend, but Alicia already has a best friend, Betty Hill, from West Tower.
At school, the headmistress, Miss Grayling, urges the new girls to leave Malory Towers one day with eager minds, kind hearts, and a will to help others. Darrell is deeply impressed and tells Miss Grayling that her father, a surgeon, said something similar.
Darrell settles into North Tower. Her dormitory holds 10 girls in the first form, the youngest year group at the school, including Katherine, the calm head girl; Mary-Lou, a timid girl frightened of everything; and Irene, brilliant at math and music but absent-minded. Darrell finds schoolwork easy and begins joining Alicia in amusing the class. One day Alicia, inspired by a trick her brother played, pretends to be deaf during a French lesson taught by Mam'zelle Dupont, the excitable French mistress. Darrell and Betty join in, and the whole class becomes a shouting chorus until Miss Potts storms in. Alicia receives a scolding and extra prep, or punishment homework.
Tensions rise at the swimming pool, a natural rock pool filled daily by the tide. Gwendoline spitefully holds the terrified Mary-Lou underwater. Darrell, shaking with rage, rescues Mary-Lou and delivers four stinging slaps to Gwendoline. Katherine orders Darrell to stop, telling her she has put herself in the wrong. Later, Darrell apologizes to Gwendoline, but Gwendoline coldly rejects the apology.
Katherine calls a first-form meeting, but Darrell walks in and apologizes to Katherine first, explaining that her terrible temper is her worst fault. She reveals she has already apologized to Gwendoline, impressing everyone. Katherine then forces Gwendoline to apologize to Mary-Lou for the ducking, the first apology Gwendoline has ever given. When Gwendoline threatens to write to her mother about the slapping, Katherine says she will also write with full context, and Gwendoline abandons the letter.
Shortly after, Alicia genuinely goes deaf from swimming underwater too long, but after her earlier prank no one believes her. Mam'zelle Rougier, the sharp-tempered second French mistress, assigns her 100 lines, and Miss Potts moves her to the front row. Darrell compares the situation to the fable of the boy who cried wolf.
Meanwhile, Gwendoline schemes to torment Mary-Lou while framing Darrell and Alicia. She befriends Mary-Lou, then plants a spider in Mary-Lou's desk, timing her move so Darrell and Alicia are seen entering the classroom beforehand. When Mary-Lou discovers the spider during French class, it runs up Mam'zelle Dupont's arm, causing pandemonium. Gwendoline tries to blame Darrell and Alicia, but Katherine dismisses the accusation. Gwendoline continues sabotaging Mary-Lou's belongings throughout the term while posing as her sympathetic friend.
As half-term approaches, Darrell mentions that her mother wrote about seeing Sally's baby sister. Sally reacts with shocking intensity, going white and insisting she has no sister.
Half-term brings family visits and a swimming exhibition. Darrell is dismayed to place 10th from the bottom in form order, the class's academic ranking. Miss Potts explains that Darrell is wholehearted: When she puts energy into mischief, her work suffers. Gwendoline spends the day boasting to her mother and governess, but Darrell inadvertently exposes some of Gwendoline's lies. Mrs Rivers confirms to Darrell that Sally does have a baby sister and that Mrs Hope is worried about Sally.
After half-term, Darrell finds Sally alone in a practice room and tries to deliver Mrs Hope's message. Sally refuses to listen. Darrell loses her temper, and when Sally pushes her, Darrell shoves back hard. Sally falls against a chair and clutches her stomach in pain. That evening, Sally does not appear at supper. Mary-Lou finds her groaning in the dormitory, and Miss Potts and Matron, the school's nurse-like caretaker, are summoned. Sally is moved to the san, the school's medical facility.
Darrell is consumed by guilt, certain her push caused Sally's illness. Too frightened to confess, she recognizes herself as a coward for the first time and gains new sympathy for Mary-Lou's habitual fearfulness. She writes a long, honest letter to Mrs Hope, confessing the quarrel and describing Sally's denial of having a sister. Sally's condition worsens, and her parents are sent for.
Unable to sleep one night, Darrell hides near the san corridor and falls asleep on a window seat. She wakes to hear her own father's voice. Mr Rivers has been called to perform an emergency appendectomy on Sally because the school's regular surgeon is ill. The operation is successful. He reassures Darrell that appendicitis cannot be caused by a push; Sally's condition had been developing for a long time. Darrell promises her father she will learn to master her temper.
The next day, Miss Grayling meets Sally's parents. Mrs Hope reveals that Sally changed completely when Baby Daphne was born after 12 years as an only child. Sally was sent to boarding school because the baby required constant care, but she interpreted this as being replaced. At Miss Grayling's suggestion, Darrell visits Sally first and tells her that Mrs Hope left the baby behind to rush to Sally's side. This transforms Sally, and when her parents arrive, she tells her mother to go home to Baby Daphne, showing her jealousy has dissolved.
As Sally recovers, she and Darrell become close friends. Sally devises a plan to boost Mary-Lou's confidence: Darrell will pretend to have a cramp in the deep end. When the plan is enacted, the life belt has been sent for repair. Without hesitating, Mary-Lou jumps fully clothed into the deep water, barely able to swim, and struggles to reach Darrell. Other swimmers pull her to safety, and Mary-Lou becomes the heroine of the school.
Gwendoline, consumed by jealousy, smashes Mary-Lou's treasured fountain pen and frames Darrell by smearing violet ink onto the sole of Darrell's shoe, hiding her own stained shoes in a cupboard. Most girls turn against Darrell; only Mary-Lou and Sally refuse to believe she is guilty.
That night, Mary-Lou, though terrified of the dark, creeps alone to the cloakroom and discovers Gwendoline's stained shoes and the ink bottle. The next morning, she presents the evidence. Gwendoline's guilty face confirms everything. Each girl apologizes to Darrell. Alicia asks to resume their old companionship, but Darrell declines, choosing Sally and Mary-Lou, the friends who stood by her.
Darrell tells Gwendoline she will keep the incident quiet if Gwendoline buys Mary-Lou a replacement pen. Exams arrive, and Darrell does very well, having redirected her energy into her studies. On the last day, Darrell rides the coach with Sally beside her, whispering a private farewell to Malory Towers. Sally will visit during the holidays, and Darrell will visit Sally and meet Baby Daphne.