The River Heights High Wildcats are facing their bitter rivals, Red Rocks High, in a girls' softball play-off game. But before the first pitch is thrown, the spirit of foul play intrudes. Someone breaks into River Heights High and steals the school's prized trophies—a treasured portrait of a pioneering woman coach and the team mascot's cougar costume!

Nancy, however, is convinced that the theft is not a simple case of bad sportsmanship. For the closer she comes to the truth, the more dangerous the game becomes. The burglar has already demonstrated a talent for disguise and deception...and has now decided to play true hardball, taking aim at Nancy Drew!