"Moon Serpents!" Astronaut Bud Barclay gasps into the microphone of his space suit. Tom Swift Jr.'s investigation of the phenomenon reveals that the giant, writhing reptilian forms are caused by gas vapors. When Tom captures a sample of the gas in a metal flask for analysis, he shoots up from the moon's surface into space! Through quick thinking the young scientist-inventor rescues himself and realizes that he has discovered a new powerful energy, which he calls Serptilium.

At this time, a large railroad network is in the market for an advance method of rail travel. A contract will go either to Swift Enterprises or to a rival firm, Cosmoprises -- whichever designs the best super-speed train.

How Tom, using Serptilium, develops his invention and defeats Cosmoprises' evil attempts to win the prized contract makes exciting reading for all Tom Swift Jr. fans.