In the frontier village Vetrov boys from the children's home called Indians lead a bitter "war" with the local Símáks. Before a battle the Indians have a meeting in the sandpit with a smoke signaling. The smoke brings to the sandpit a frontier guard Tomás with a police dog Ranek. The children learn from Tomás that Rank served out and that it shall be destroyed. The nine-year old Fanda writes a letter to the "minister of the frontier guards" in which he asks to let Ranek live. To save Ranek's life he also visits the local frontier guard commander. In the end it is decided that children from the children's home may keep Ranek.

This tale is about young Ludvik (Robert Nespor) an erstwhile, budding scientist whose practical sense leads him to experiment before his common sense has figured out the results. Due to that unfortunate pattern, he variously floods the bathroom while working on displacement of volume and weight in water, he shatters the harvest pumpkins but not the law of gravity, and now he is eyeing the neighbor's rabbits for an electricity experiment. While the rabbits remain safe for awhile, Ludvik's parents wish he would follow in their more traditional footsteps and dedicate himself to music. Instead, Ludvik has learned that the proper mixture of a certain carbide and water can be explosive, and while he works on that technique, a friend tips off the physics teacher about these potentially disastrous plans.