Three siblings — Ross, Wendy, and Diccon Carsedale — are thrilled to spend their summer holiday at their aunts' home in Scotland. The children quickly befriend local girl Lettice and her cousin Malcolm, forming a merry band of adventurers.

Together they explore mysterious ruins, go on daring seaside escapades, and uncover long-buried secrets. But when the children's curiosity leads them to a forbidden wing of the house, they learn a painful family history. As summer progresses, the young friends must navigate pranks gone awry, dangerous rescues, and the complexities of growing up, all while savoring the beauty and wonder of the Scottish countryside.