To celebrate her birthday, George is taking Nancy and Bess on a bike tour around Mystic Lake, but when their tour guide disappears and unknown adversaries steal their supplies and slash their tires, Nancy begins looking for clues.
When competition for lucrative prizes ranging from money to placement on the Olympic Team escalates to the point where one desperate participant drugs horses at the River Heights Horse Show, Nancy, Bess, and George track clues to identify the culprit.
When Nancy, Bess, and George continue their journey and board a train bound for Denali National Park, they become aware that danger has followed them and Nancy worries her undercover status has been discovered.