Along with a high desire for adventure, a passion for tennis quickly bonds four girls to each other. Teammates Carol and Olive befriend Pam Stewart, their tournament rival, and Carol invites her to spend the school holiday with them at her family’s Tudor mansion. Pam’s sidekick and junior Glenda Welland is also invited, much to the younger girl’s ecstatic delight. Between matches, the quartet thoroughly investigate the treasures of the Adams house, including hidden passages, priest-holes, and a rare Thomas Malory manuscript owned by Carol’s father. After overhearing a conversation of sinister bent, Pam grows concerned that some of the houseguests may be plotting to steal the precious document.
Summer vacation finds the friends reunited once more, with Pam still wary that the manuscript is in danger. Upon Pam’s urging to ensure the papers are genuine, Mr. Adams calls upon the opinion of Sir Selim Cortez—who promptly identifies them as forgeries. Disheartened, Mr. Adams bequeaths the manuscript to Pam, which turns out to be a good decision: the resourceful young lady must use her wits to keep ahead of the thieves, who ruthlessly chase after the “useless” papers. Pam and the girls manage to enjoy all aspects of their holiday—including river boating, sight seeing, a formal ball, and, of course, a tennis tourney—while keeping the valuable Malory script safe and unmasking the scheming villains in the process.