Alrik falls through the frozen water during his school's annual ice sports celebration on Lake Malaren. People blame it on an evil aquatic creature that lives in the depths. At the same time, Viggo's and Alrik's mother shows up with an amazing secret and a new guard appears at Castle Gripsholm. Are these events related? In a place like Mariefred, the consequences could be terrifying! Un terrible accidente ocurre cuando el colegio de Viggo y Alrik celebra la jornada anual de deportes de hielo en el lago Malaren. ¡El hielo se rompe y Alrik se hunde en el agua helada! Sospechan que ha sido obra de una perversa criatura acuática que acecha en la profundidades.....