On the distant mining world of New Aries, a young colonist, Jim Marlowe, has acquired a native pet, a "roundhead" he names Willis, which can parrot speech and record visual information. As Jim and his sister P.J. are about to be sent to a boarding school, their mother Jane, the colony's chief medical officer, discovers that a substance in the deeper mines is killing the miners. She uses her authority to shut down the mines, but the decision is rescinded by the evil colony leader, a Beta Earth Mining Company man who sees only the rich profits at any cost. When the colony leader and the school's headmaster discover that Jim has a roundhead in captivity, they seek to steal it for use in medical experiments to make a serum that will temporarily protect the miners against the deadly substances in the mines. Willis records their conversation and when Jim escapes with him, it sets off a revolt against the Company that ends up involving the mysterious intelligent natives. Only Jim's friendship with Willis saves the colonists from eviction or possibly genocide at the hands of the natives, and in an epilogue set many decades later, we see that humans remained and "terraformed" New Aries to make it suitable for both species to co-exist.