The film tells children in an entertaining way how to behave while swimming in the river, using the recommendations of the Society for the Rescue of Waters (OSVOD). Two friends come to the river to swim. They play in the water, arrange a "sea battle". With the help of "magic" the children get a big book "The ABC of Neptune". Using this book, you can not only learn to swim, but also learn the rules of behavior on the water. During the game, one of the boys drowns, but the alphabet contains instructions on how to provide first aid. This useful book was given to the children by "Neptune" at their holiday, which was arranged for children studying in OSVOD (Society of Rescue on Waters). There, children learn to swim and dive under the guidance of a trainer on a safe, fenced-in section of the river.