Trurl and Klapaucius are brilliant (robotic) engineers, called "constructors" (because they can construct practically anything at will), capable of almost God-like exploits. For instance, on one occasion Trurl creates an entity capable of extracting accurate information from the random motion of gas particles, which he calls a "Demon of the Second Kind". He describes the "Demon of the First Kind" as a Maxwell's demon. On another, the two constructors re-arrange stars near their home planet in order to advertise.
The duo are best friends and rivals. When they are not busy constructing revolutionary mechanisms at home, they travel the universe, aiding those in need. As the characters are firmly established as good and righteous, they take no shame in accepting handsome rewards for their services. If rewards were promised and not delivered, the constructors may even severely punish those who deceived them.