Natka, who dreams of becoming a captain, is sent to pioneer work. She takes her first assignment very seriously - and receives a trip to Crimea. In Artek, Natka is asked to replace a sick pioneer leader. The girl agrees. Soon, during the construction of a water pipeline, a dam breaks and a struggle for water begins. Engineer Ganin, who came on vacation with his little son Alka, also joins this fight. In the ruins of the old fortress, Vladik and Tolik find dynamite. To track down their enemies, the guys hide explosives and swear not to reveal their “military secrets”...