Incomprehensible things are happening at the hippodrome. The winner of the latest race, a bay horse named Dark, behaves strangely - either he is among the leaders, or he shamefully trails behind the race. And then Dark’s double appeared. This horse is just as beautiful, strong and has the same strangeness - sometimes it wins the race brilliantly, sometimes it loses it outright. What is happening? Something is clearly wrong here. Three friends - Tracy, Holly and Belinda - set out to solve the mystery of the hippodrome. The members of the Detective Club have successfully solved many crimes. Will they be able to find out the reason for the mysterious victories and defeats of prize horses or will the young detectives be destined to fail this time?..