Tom Ripley is asked to get his aquintance Dickie Greenfield back to Europe; and if you don't want to be spoilered, don't read further

An envoy gets sent to a winter planet to talk its people into joining a commonwealth of worlds. The catch is the people: Gethenians have no fixed sex. They're neutral most of the month and only become male or female during kemmer, and it isn't the same one every time.