At the water sprite and water nymph school, classes proceed as planned until the water nymphs leave for a dance competition and the water sprites accompany them to cheer them on. Only Vrbata, a notorious troublemaker, remains at the school as punishment. While exploring the cabinet, he carelessly awakens an octopus with living water. Upon the school trip's return, the octopus manages to capture the headmaster and begins to threaten the entire school. At the same time, the vagrant Mikolášek breaks into the school, so the water sprites, water nymphs, and teaching staff must not only fight the sea monster but also solve the problem of how to get rid of him. Unfortunately, nothing goes according to plan, and the octopus gradually captures everyone except the clumsiest water nymph, Křehulová. The situation begins to look hopeless. Fortunately, Mikolášek comes up with a plan and, together with Křehulová, tries to bring it to a successful conclusion...