The book is set in the constellation Hydrus, which is known in the Alliance-Union universe as the "Hydri Reach", and also as "Serpent's Reach", from which the book takes its title. The Hydri stars are home to the alien "Majat", an insectoid race with a hive-mind consciousness. Like some species of Earth insects, the Majat are eusocial, featuring separate Drone, Worker and Warrior castes that serve each hive's single Queen. There are only four distinct hive-mind identities in the Majat species which were arbitrarily assigned colors by the humans: blue and green (the "tame" hives), and red and gold (the "wild" hives). They are native to Cerdin, a planet in the Alpha Hydri system (but see "Majat planet of origin" below).