Natka, who dreams of becoming a captain, is sent to do pioneer work. For her serious approach to her first assignment, she receives a trip to Crimea. In Artek, Natka is asked to replace a sick pioneer leader. She agrees. Soon, a dam breaks during construction of a water pipeline, and a fight for water begins.
Engineer Ganin, who has come on vacation with his young son Alka, also joins this struggle. In the ruins of an old fortress, Vladik and Tolik find dynamite. To track down their enemies, the boys hide the explosives and swear not to reveal the "military secret"...
