The latest adventure for brilliant, modest Lucky Starr, agent for Earth's Council of Science some centuries hence, starts the minute Lucky sets foot on Mercury to investigate the mysterious accidents gumming up Project Light. Project Light has to do with the principle of hyperspace, an arrangement whereby things are freed to travel faster than the speed of light. Using this fact, Earth can establish a series of space stations to regulate the sun's rays so that it will have light and good weather, when and where it wants it. But this regulation will, in turn, make Earth more vulnerable to attack from the warlike inhabitants of the Sirian system. In a misguided wish to prevent this, Dr. Peverale, an elderly astronomer, is using a Sirian robot which he had previously acquired, to stop Project Light. Lucky intervenes in time.