A sixth-grader with an unusual gift and an equally unusual name, Cassandra, moves to another city after her parents’ divorce. There she notices strange things happening in her new lyceum. Some in the class are hostile towards Kasa. She begins to make friends with Nika and Petya, they take part in a theater production and follow a strange supply manager who seems to be stealing equipment donated by the lyceum. Adults don't believe them. On the Day of the Lyceum and the premiere of the literary performance, thanks to the gift of Cassandra and the courage and ingenuity of Nika and Petit, the trio uncover a gangster conspiracy and a mysterious crime committed in their lyceum.