Jakob Faraday is exploring Jupiter when his small craft is suddenly jarred by contact with a living creature. Faraday realizes he has blundered into a herd of enormous manta/dolphin-like animals, the Qanska, which live in Jovian space. To learn more about the Qanska, Faraday challenges Matt Rainey, a recently crippled quadraplegic, to bond with a Qanska by being placed within the womb of a mother Qanska. Rainey accepts, becoming a unique hybrid, still human in personality, but physically a Qanska.

Then a rumor spreads on Earth: the Qanska have a secret stardrive which could take Earth vessels to the far reaches of space. Faraday is removed from control of the Jupiter operations and replaced by Arbiter Liadof, who tries to coerce the Qanska into sharing their secret.

Rainey knows there is no drive. Worse yet, the Qanska environment is inexplicably failing, as has happened before in previous herd locations. Rainey might be able to save the Qanska if he can prevent the destruction of their environment. But Rainey has unwittingly violated a Qanskan code, and he is to be banished, no matter that he is the giant creatures' last hope.