The Starmen have learned that a mysterious alien race visited our Solar System thousands of years ago. In the third Starman book, Journey to the Tenth Planet, Zip Foster called them "The Benefactors" because of the kindness these people showed the diminutive people who live on Titan. While they were on the tenth planet, the Starmen discovered that the Benefactors have a vicious implacable enemy the Xenobots, a violent race that is searching for its "ancient enemy," and will demolish Earth if necessary to further the search. In Descent Into Europa, Zip Foster organizes a desperate search to find the Benefactors before the enemy does. Without their help, Earth stands no chance against the technologically superior Xenobots. With impeccable logic, the Starman leader has determined that if the Benefactors had a central base in the Solar System, it had to be on Europa, one of the moons of Jupiter. What the Starmen do not even suspect is that there is also a powerful enemy close to home.