Bibi & Tina secretly raced at night. When they return to the Martinshof, they watch as a veiled rider on a ghost horse scares the holiday children! Immediately they begin the chase, but they can only capture a white sheet that the rider loses. Ms. Martin asks Bibi & Tina to find out who the mysterious stranger is. The trail leads first to Mühlenhof, but the Mühlenhofbauer is innocent. Then Alex becomes a suspect!