Ten-year-old Mandy has lived at St. Martin's Orphanage, a large white house on the outskirts of a country village called St. Martin's Green, for as long as she can remember. She has no known relatives. The orphanage is run by Mrs. Hannah Bridie, an elderly widow who has served as matron for 20 years. Mandy is bright, inventive, and a dreamer. The staff grants her more freedom than the other children, yet she is plagued by recurring bouts of deep, unexplained sadness, as though she is searching for something she cannot identify.
The high stone wall at the back of the orphanage orchard fascinates Mandy. One afternoon, she climbs it and descends into a vast beech wood. Following a footpath to a sunny clearing, she discovers a very old, small cottage in disrepair. Inside, past an empty main room, a tiny kitchen, and an upstairs bedroom, she finds a room entirely decorated with seashells, with gold-painted woodwork, a tile or marble floor, and a large fireplace. Astonished, Mandy resolves to adopt the cottage as her own secret place, cleaning the interior and tending the garden.
She begins working without telling anyone, borrowing tools from Jake, the orphanage's elderly handyman, and gathering supplies from the kitchen. Ellie, a maid at the orphanage, confirms that the land beyond the wall belongs to a large estate where nobody lives, reassuring Mandy that her cottage is unclaimed. She tells no one, not even Sue, her close friend and roommate.
On Saturday, after her shift at the local grocery store, Mandy visits a garden supply shop. The proprietor, Mr. Simple, generously gives her about 30 seedlings, charging her only for a watering can. She borrows a rope from Jake's shed and hoists the box over the wall, narrowly avoiding discovery when Sue wanders through the orchard. At the cottage, she plants the seedlings and waters them from the nearby stream.
Over the following weeks, Mandy secretly takes cutlery and dishes from the orphanage for the cottage. She sweeps and scrubs the interior, cleans the shell room, and washes the curtains in the stream. When she tries to light a fire, she discovers the chimney is blocked by an old bird's nest and climbs onto the roof to remove it.
Matron Bridie, concerned about Mandy's prolonged absences, summons her. Panicking, Mandy lies and claims she has given up her project. Matron offers a garden plot at the orphanage, but Mandy declines, leaving in tears. Sue grows resentful of Mandy's secrecy and one day follows her over the wall. Mandy diverts Sue away from the cottage path, and the alternate route leads them to a wrought-iron gate that opens onto the main road. Its padlock is unlocked, giving Mandy an easier route to the cottage.
By July, Mandy's garden is a mass of color, attracting wildlife including a sparrow she names Chip and a young deer she names Snow, after her childhood fantasy of a white unicorn. However, her conscience troubles her: She has been stealing small items from the orphanage. Matron assembles the children and announces that things have gone missing. Guilt-ridden, Mandy returns everything and begins working overtime at the grocery store to buy her own supplies.
When Mandy wants to trim the cottage hedges, Jake refuses to lend his shears. During the annual Bank Holiday seaside trip, she fakes illness to stay behind and takes Jake's shears without permission. She clips the lawn but forces them on a thick branch and breaks them apart. Matron summons Mandy, who admits breaking the shears but refuses to explain. She is confined to the orphanage for one week and must save her pocket money for replacements.
In autumn, mysterious signs appear. Mandy finds boot prints outside the cottage and a horseshoe print nearby. The iron gate is locked with a new padlock, forcing her to climb the wall again. Her broken garden gate is mended with a new latch, and the entire garden is cleared of weeds, the hedges trimmed, and the lawn clipped by an unknown hand. A note nailed to the doorjamb reads, "FOR THE LITTLE GIRL WHO COMES HERE. HOPE YOU LIKE THE GARDEN," signed "AN ADMIRER" (152). Mandy is overjoyed and becomes convinced the mysterious benefactor is the prince she has always imagined.
Her excitement peaks when she finds a chrysanthemum plant at the cottage with another note from AN ADMIRER. At that moment, Sue appears, having followed Mandy from school. Mandy hides the note but cannot undo Sue's discovery. Back at the orphanage, Mandy's fever becomes apparent. Matron orders her to come straight home after school and to stay near the orphanage on weekends until warmer weather returns, devastating her.
Desperate to leave a note for AN ADMIRER, Mandy sneaks out during a violent storm. Soaked and feverish, she reaches the cottage but is too ill to write. Hallucinations overwhelm her, and she collapses on the floor, crying out for her mother.
At the orphanage, Ellie discovers Mandy missing. Sue reveals the cottage's existence to Matron, who calls Bill Fitzgerald, the new owner of the estate. Bill saddles his mare and rides into the storm. He finds Mandy collapsed and delirious, wraps her in his coat, and carries her on horseback to Cranton House, his ancestral home.
Mandy develops pneumonia and is delirious for three days. Ann Fitzgerald, Bill's wife, stays at her bedside. Gradually Mandy's fever subsides. She meets the Fitzgeralds properly and learns that Sue's knowledge of the cottage saved her life. As Mandy recovers, she grows deeply attached to Bill and Ann. Their 14-year-old son, Jonathan, arrives home from school. Though Mandy is initially intimidated by his energy, Ann helps her see that Jonathan is probably nervous about her too. An evening where the family dines together and Ann plays a hidden pipe organ in the library becomes one of the happiest moments of Mandy's life.
Bill reveals that he is AN ADMIRER. He saw Mandy working in the garden while riding around his estate and chose not to disturb her, leaving notes as a playful mystery. Overwhelmed, Mandy weeps. When Bill asks what is wrong, she whispers one word: "Snow" (256), connecting the name of the deer to her deepest longings for love and belonging.
Matron delivers an invitation to spend Christmas at Cranton House. Christmas is magical: a candlelit dinner, carols by the organ, and generous gifts, including a red dress, a gold signet ring, and a teddy bear from Jonathan. But when Matron collects Mandy the next day, her return to the orphanage plunges her into despair. She avoids the Fitzgeralds' phone calls because speaking to them is too painful.
Convinced she cannot endure her old life, Mandy tells Sue she plans to run away. Sue reluctantly helps her pack. But that very night, before Mandy can leave, Bill and Ann arrive at the orphanage. They have petitioned the authorities for permission to adopt Mandy, and approval has come through. Ann asks if Mandy would consider coming to live with them as a family. Speechless at first, Mandy is flooded with happiness. She goes into Ann's arms and both weep with joy.
Mandy waits until morning to leave, unwilling to abandon Sue abruptly. The entire orphanage gathers to say goodbye. On the drive to Cranton House, Bill shows Mandy the estate and offers her Shell Cottage, the shell-decorated cottage she discovered. Instead of keeping it, Mandy asks that it be given to the orphanage children as a place to play and garden. Bill is moved and agrees. As the car pulls up and Ann and Jonathan greet her at the door, Mandy realizes she has found what she has been searching for all her life: a home, a family, and people to love and be loved by.