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 She uses her authority to shut down the mines, but the decision is rescinded by the evil colony leader, a Beta Earth Mining Company man who sees only the rich profits at any cost.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.