When documentary filmmaker Penny Lane chose to donate a kidney to a stranger — the dramatic crux of her new “Confessions of a Good Samaritan” — one can’t help but wonder whether the movie idea came first or the altruistic impulse. Because really, who does that?

Lane, one of our most playfully inquisitive truth-is-stranger-than-fiction filmmakers, is fine with that speculation, because “Confessions” is as much about the mysteries of our human impulses as it is about elective surgery and the roots of altruism. With over 100,000 people in need of a kidney, there’s a gap to explore between an obviously solvable problem and the fact that people aren’t exactly busting down hospital doors to give up a kidney they don’t need.

It’s a conundrum ripe for Lane’s offbeat approach to nonfiction. And in turning the camera on herself, after focusing on eccentric subjects ranging from an infamous quack (“Nuts!”) to religious liberty (“Hail Satan?”) and polarizing music taste (“Listening to Kenny G”), Lane proves to be a charmingly honest tour guide for her own journey into another peculiar corner of the world. Along the way she includes the voices of other donors and a handful of engaging professionals (a cheerleading transplant surgeon, a dedicated neuroscience professor, a jokey bioethicist) who provide a wider view on the morality, emotions and history of kidney donation.

As it happens, Lane does tell us early on that her decision to give preceded the decision to film. What makes it believable is the arc of research and self-analysis we’re privy to over the course of “Confessions.” Watching what Lane goes through, it’s clear this is a complicated commitment for her. In one of her on-screen confessionals, Lane, with surgery a week away, admits that her anxiety and hopelessness are hitting all-time highs. But we also hear how emboldened she is by the lifesaving logic of it all, that she’s participating in humankind at its humankindest.

And yet, as her dive into the procedure’s history shows, resistance has been baked into transplant surgery since the beginning. It was experimental (and called unethical in the medical community) for a long time, until it was safe between twins, then safe between the unrelated with the help of immunosuppressing drugs. But when “good Samaritan” donations became more well-known — thanks to countless news segments, thick with bite-sized uplift — other questions arose about how this charitable act fits into our wider society. You may not be judged for keeping your kidneys while participating in all manner of risky behavior, but wouldn’t a healthy twin with an ailing sibling be?

If you ask advocate Sally Satel, also featured in the film, the obvious answer to all these thorny questions is compensation, as controversial as that notion is to many. (The name of Satel’s book-long proposal: “When Altruism Isn’t Enough.”) As Lane wends her way to a conclusion of her own, it’d be a mistake to view “Confessions of a Good Samaritan,” with its serious curiosity balanced by a quirky theremin score and humorous asides, as Lane turning personal experience into public advocacy. She’s cagier than that, preferring a messy, sometimes humiliating, sometimes exhilarating ride-along as its own worthy tale of the pitfalls and pleasures of willful, anonymous neighborliness. It doesn’t look easy. It does look incredibly rewarding.