Maria Merryweather inherits an intriguing tome following her father's passing: The Ancient Chronicles of Moonacre Valley. In her readings, she learns about the first Moon Princess who received mystical pearls from the moon itself. During the princess's wedding to a Merryweather, her father, a de Noir, gifts them a black lion, while the groom presents his bride with a unicorn. However, as the princess unveils the pearls, greed overtakes the families.
Forced out of her London residence, Maria, accompanied by her governess, Miss Heliotrope, journeys to the ancestral home of Moonacre Manor to reside with her estranged uncle, Sir Benjamin Merryweather. Upon arrival, Sir Benjamin warns her about potential dangers, specifically bandits, and urges Maria to steer clear of the forest and the rival de Noir family. During an outing with her uncle, she hears a cry for help and ventures into the woods, where she is confronted by bandits, including a boy named Robin de Noir. Just as they plan to abduct her, Sir Benjamin’s fierce black dog, Wrolf, scares them off.
Continuing her reading, Maria uncovers that a fierce rivalry between the two families over the pearls led to the Moon Princess cursing Moonacre Valley, vowing eternal darkness at the rise of the 5000th moon unless a "pure heart" intervenes. The local chef, Marmaduke, reveals to Maria that she is destined to be the new Moon Princess since only she can see the unicorn, the key to breaking the curse. Discovering that the impending full moon marks the 5000th moon, Maria flees and encounters a woman named Loveday in a cave. Loveday, once a Moon Princess herself, discloses her failure to unite the families and lift the curse. Realizing the de Noirs have absconded with the pearl casket and the Merryweathers hold its key, Maria finds the key concealed within her book's bookmark.
Determined, Maria discreetly enters the de Noir fortress but gets caught by Coeur de Noir, Robin's father. After reluctantly handing him the key, he accuses the Merryweathers of stealing the pearls when he discovers the casket is empty. Maria narrowly escapes thanks to Wrolf. Back at the manor, she finds Loveday, who provides more insights into her familial history: Loveday and Sir Benjamin were once engaged until she revealed her lineage as a de Noir, resulting in their bitter split.
Maria notices a portrait of the first Moon Princess, which guides her to where the pearls are hidden within the forest. Using trickery, she sends forged letters to Sir Benjamin and Loveday, arranging for them to meet and begging Robin to assist her. Unbeknownst to them, their meeting only reveals Maria’s deceit. Together with Miss Heliotrope, Marmaduke, and the simple-minded servant Digweed, they set off to find her.
Meanwhile, Maria and Robin locate the pearls hidden in a hollow tree and make their escape through a secret tunnel, but not without incident – Coeur’s men pursue them, resulting in the tragic death of Wrolf. Just as the cursed moon is about to rise, Coeur reaches the cliff where the affliction began. Moments later, Sir Benjamin appears with Digweed and Marmaduke, followed by Maria and Robin. Loveday, now revealed as Coeur’s daughter, also joins the tense standoff. Maria desperately pleads with her uncle and Coeur to set aside their longstanding grudges to lift the curse, but her words fall flat. Realizing she alone must act, Maria plunges into the ocean, pearls in hand.
In a spellbinding moment, a massive wave crashes against the cliff as Maria emerges riding the majestic unicorn, with Wrolf having transformed back into his true form: the black lion. The families reconcile, Miss Heliotrope arrives just in time, and the curse is finally lifted. With Moonacre Valley restored to its former glory, love blossoms between Maria and Robin, completing their extraordinary journey.
