**SPOILER AT THE END**
It's easy to see how this book became a classic; it's mind-boggling, engaging, infuriating, and eye-opening. Though it's labeled science fiction, it really defies such classification. "Stranger in a Strange Land" is equally science fiction and philosophy. It uses fiction as a means of holding up a mirror to ourselves, and what we see, what Heinlein saw, wasn't pretty.
The plot is this: Valentine Michael Smith is a human being born on Mars and raised by Martians. He is discovered and "rescued," and begins to learn what it is to be human. And he does, eventually, but not without first changing utterly and completely those he comes in contact with. He is a Superman: beautiful, highly intelligent, and capable of producing what average men call miracles in the blink of an eye. He attains the Truth - "Thou art God" - and goes about trying to educate the human race.
The result is this, as witnessed through the many philosophical discussions in which the characters take place: man is a brutal, unkind species. He has become so infatuated with hate and differentiation and violence that he cannot even recognize it in himself. Humans have so utterly stigmatized the only true act of love (sex) that there is room for nothing but "wrongness." The Man from Mars reenacts the story of Jesus, who also died in giddy violence all while preaching love. The ending message is one of hope, albeit dim, that eventually humanity will weed out the wrong and embrace the good, embrace oneness and love. But the path to goodness is always riddled with martyrs.