Ten-year-old Jock Avery is convinced his young friend's imagination is working overtime when he claims there is a mermaid in their lake. Soon learning what doubt can do to a friendship, Jock and his two new friends, Lynna and Chip, set out to find this mermaid. What they find may surprise even them: doubting - and seeing - are two different matters, yet both are tied to strong friendships like a knot on a fishing hook.