Mark Fraser was a scientist, not a soldier; he had chosen his life as a communications specialist on Ganymede, his wife and children with him, his project an attempt to reach and talk with the strange methane-and-ammonia quadrupeds who lived on the high-pressure hell of Jupiter's surface. When a renegade Earth traitor landed his ship on Ganymede, Fraser found himself involved in a sudden, suicidal war. His fellows were defeated. Earth itself was threatened. Yet Fraser found his thoughts going out to Theor, centaur-like in body, his metabolism a sink of poisons, his thoughts utterly alien to anything human. . . . yet a friend to Fraser, and like Fraser the defeated in a sudden war. For while Ganymede had been captured by treason, on Jupiter itself Theor's people had been overrun by larger, stronger creatures migrating in search of land to till and cities to loot. When all seems lost, can these two utterly alien friends hold the key to each others' salvations?

This is the original version of Three Worlds to Conquer, as serialized in "If, Worlds of Science Fiction" during the fall and winter of 1963.