Genly Ai, an envoy from a league of worlds called the Ekumen, comes alone to the frozen planet Gethen, known as Winter, to invite its nations to join. Gethenians have no fixed sex, becoming male or female for a few days each month, and Ai's struggle to understand them, above all the exiled politician Estraven, the one who trusts him, leads to court intrigue and a long flight across the ice.