The Cluster is a vast alien machine that destroys starships indiscriminately in its quest for something or someone. Commander John Mark Ellis, disgraced and booted out of the service when he fails to save a merchant ship, believes the key to stopping the Cluster is communication. Clyde McClintlock believes the Cluster is God incarnate. G'Liat is an alien warrior whose own starship was destroyed by the Cluster. All together these three set out to solve the mystery of the Cluster before it finds the object of its quest.