Remember Sonny and Cher's cute little blonde girl, Chastity, from their '70s variety show? And the kerfuffle when she grew up and came out as a lesbian (and Cher lost it)? Well, she is now a he -- a beefy man named Chaz.

The journey from there to here couldn't be more moving in this astonishingly intimate documentary from the World of Wonder production team that's taken us inside the singular worlds of Tammy Faye Bakker and Monica Lewinsky.

We meet Chaz in hormone therapy before surgery to have his breasts removed; he already sounds amazingly like dad Sonny Bono. Chaz lets the cameras shoot everything, whether he's meeting doctors, visiting his aunt or squabbling with his girlfriend.

So it's an all-access pass -- and not just to Chaz, but also to his love of five years, Jennifer, a lesbian who fell for a woman who's about to become a man. In other words: Two identities get redefined here.

"Under the best of circumstances, relationships are tough," Chaz says. "Throw in substance abuse and a sex change, and it gets a little bit more complicated."

MY SAY Sex change! Substance abuse! Paparazzi! Cher! This could have been a tabloid field day. But "Becoming Chaz" is an immersive eavesdrop with two people so open and sincere, you can't help but fall in love with them -- or panic when their couplehood is threatened: on Chaz's side by Jennifer's relapse into substance abuse while he stays clean, and on her side by his sudden surge of "overpowering male energy. I miss the person I met five years ago."

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Need more drama? Here comes Cher, clearly still creeped out by it all.

BOTTOM LINE As real as real gets, invaluably adding human understanding to a hot-button topic.

GRADE A