Long, long ago, lived a handsome young tsar named Peter. When he decided to marry, he rejected the beautiful Molnya because of her cold, hungry eyes. But Molnya was really a witch, and she put a curse on Peter: "When you find your hoped-for bride, your ruin will be at hand."

When Peter goes off to marry the Little Sister of the Sun, Molnya disguises herself as a baby and gets taken into the tsar's castle. The "baby" soon grows to a monstrous size, destroying everything in sight, and Peter must hurry home to save his kingdom. With the help of three friendly giants and a little mouse, Peter escapes the giant witch-baby and lives to marry the Little Sister of the Sun and rule wisely and well.