Genly Ai, Terran Envoy, is on a mission to the isolated planet of Gethen in hopes of forming an alliance for trade and diplomatic relations. But Gethen is unlike any other planet in the solar system - not only is the climate harsh and unforgiving (previous explorers nicknamed it "Winter"), the people of Gethen are different too - rather than having males and females, there is only one sex with the potential for both. The genderlessness of society is more powerful than you would imagine, and Mr. Ai must overcome his culture shock and learn to understand the Gethenians before he can fulfill his mission.
This is a bit of a tough read. Le Guin writes clearly and beautifully, but the vast differences between our society and Gethen are mindblowing. She has thought of everything in creating this world - language, culture, naming conventions, table manners, the calendar system - and manages to incorporate them into the story to make it seem so real, as if this were actually field notes from an interstellar diplomat rather than fiction. However, it is a double-edged sword, because there is a lot of information to keep track of and all those cultural tidbits and customs inform the story. To add to the confusion, the perspective alternates between characters, and the chapters are also interspersed with Gethenian legends and historical notes.
It pays off though, and The Left Hand of Darkness is worth a read. It is a fascinating speculation of a genderless society and will definitely make you think. Not only is it a classic piece of science fiction, it should also be required reading for anyone interested in women's or gender studies.