It's the smiles you notice first: three big, beaming, genuinely infectious grins that spread from ear-to-ear and then on to the faces of anyone else who sees them.

You'll see those smiles repeatedly in the first 30 minutes of Three Identical Strangers, this year's must-see documentary at the New Zealand International Film Festival.

That means it often feels like you're watching the happiest doco ever as it follows three identical triplets - Bobby, Eddy and David - who were separated after their birth in New York.

Twenty years later, in 1980, they reunite through random circumstances and become mass media darlings. Newspapers love them, television hosts adore them, chat shows reel them in - and why wouldn't they? They're incredible talent: funny, with the same quirks and mannerisms, often finishing each other's sentences.

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The camera loves them, and thanks to all that joyous news footage, the vibe for the first third of Three Identical Strangers has "happy ending" written all over it.

But, as one talking head says in this exquisitely made and well-told story by director Tim Wardle, "that's when things got funky".

To spoil what happens next would be doing a disservice to a film where the less you know going into it, the better.

Rest assured, as this at-first happy story morphs into something far deeper and darker than it ever seems possible, you're guaranteed more twists and turns than any fictional film in recent memory.

Go in cold, and you're likely to emerge with your blood boiling. This has to be seen to be believed.

Three Identical Strangers

Director: Tim Wardle
Time: 96 minutes
Verdict: Three's not a crowd in genuinely shocking doco

* Three Identical Strangers is screening as part of the NZIFF. Check the website for screening details.