The story follows a young boy named Leo Colston, who in the year 1900, is a guest of Marcus Maudsley, his wealthy school friend, as Leo spend the summer holidays at his family's Norfolk country house. While there, Marcus becomes sick, and he is quarantined with the measles. Left to entertain himself, Leo befriends Marcus's beautiful elder sister Marian Maudsley, and finds himself a messenger, carrying messages between her and Ted Burgess, a tenant farmer, with whom she is engaging in a clandestine affair.