It is 1842 and Maria Merryweather, a thirteen-year-old orphan, is on a journey to the mysterious Moonacre Manor somewhere in the west of England after the death of her Father, accompanied by her governess Miss Heliotrope and dog Wiggins. There she finds herself in a world out of time. Maria meets her cousin, Sir Benjamin Merryweather, one of the "sun" Merryweathers, and finds that she loves him right away - the "sun" and the "moon" Merryweathers always take to each other - for a while. Maria discovers that there is an ancient story around founding of the estate that will have a great effect on her future....
Although she is aided by a stable of wonderful characters and magical beasts (including the unicorn of the title), it is only by self-sacrifice and perseverance that she will be able to succeed in bringing wholeness back to Moonacre. And of course, with a pinch of magic, Maria is able to save Moonacre, right the wrongs, reunite lost loves and finally bring peace to the valley, and there is the happiest of happy endings.