The pioneer detachment goes on a hike through the mountains of Crimea. An enemy saboteur makes his way there. Together with his accomplices, he plans to blow up a high-mountain dam. The pioneers and their older comrades stand in the way of the enemies: teachers, security officers, collective farmers. Ahead are dangerous passes, waterfalls, and most importantly, a mysterious cave, under the arches of which a decisive battle will take place. The story “The Secret of the Moaning Cave” (1958) by Afrikan Shebalov (1923–1993) is one of the best examples of Soviet spy literature. In addition, the collection includes the story “Swallow” (1961), dedicated to the difficult service of military sailors.