I've never had such a hard time figuring out how to even approach reviewing a movie as I'm having with the documentary Dear Zachary: A Letter to a Son About His Father. It would be easy if I weren't wholly impressed with what I'd unreservedly praise as the most emotionally powerful film of 2008, and maybe the whole decade. But I can't recommend it, not just like that, because at the very same time - and for the very same reason - Dear Zachary is one of the most upsetting movies I've ever seen. Worse still, explaining why that's the case is necessarily a matter of giving away a detail that director Kurt Kuenne has presented within the film, for very good reason, as a shocking twist. Yet so important is that twist to the overall meaning of the piece that I genuinely don't know how to speak about the film in anything but the vaguest terms without spoiling it.

Which is why, with grave misgivings, I am going to spoil it (I'll put it off as long as I can), though I feel tremendously guilty about doing so and almost wish that everybody stop reading this review right now. Still there? Okay, here are the facts: in November, 2001, Andrew Bagby, a 28-year-old medical resident in Latrobe, PA, was murdered by his 40-year-old ex-girlfriend Shirley Turner. Bagby - "Bags" to his friends - was an immensely well-liked young man, and it so happens that one of his oldest friends was a filmmaker, who like every filmmaker since the art began felt the urgent need to funnel all of his feelings of grief and loss through the lens of a camera. Thus did Kuenne launch on an international road trip to interview every one of Bagby's friends and relatives that he could find, to create a cinematic portrait that would capture the memories of his dear friend forever.

At the same time, Turner fled to Newfoundland, leaving an unpleasant surprise for Bagby's parents Kate and David: she was pregnant with Andrew's child. So the bereaved parents, who had been openly considering suicide on the event of their only son's death, sold everything they owned to pay for a lawyer and lodgings for the indeterminate future in Canada, where they were warned, "the law moves slow."

Slow indeed: slow enough that Turner could give birth to a son, Zachary Andrew, long before anything approaching an extradition trial could be arranged. With Zachary's birth, Kuenne's project shifted focus, becoming a document for the newborn about his unknown father, so that when Kate and David finally got permanent custody of the boy, he'd have something to look back at as a guide to where he came from. Meanwhile, the Bagbys only got to see their grandson erratically, until that happy moment when Turner was sent to prison for the first time, and they were granted temporary full custody.

Yes, the first time: Turner popped in and out of jail a couple of times over the next year, never quite getting that extradition hearing, and making Kate and David's life a general hell. But eh, that's the Canadian legal system for you, or at least that is what Dear Zachary would have you believe (the movie is among other things, a tremendously persuasive piece of agitprop for reform of the Canadian bail system).

So far, Kuenne's film has already staked out a space as something of a small masterpiece, using found footage from Andrew Bagby's childhood alongside newly-recorded interviews with the director providing narration through an elliptically-structured but fairly coherent narrative, that is at once a loving portrait of a friend and a tribute to his wonderful parents, whose strength in an unendurable time is literally incredible. Dear Zachary is at heart a part of Kurt Kuenne's grieving process, and it is a beautiful and brave thing that he has done to let us inside his very soul during what was surely the worst time of his life - the film is a one-of-a-kind artifact, in that way, and it is deeply moving.

If I were playing by the film's rules, this is where I'd end, with a hearty and enthusiastic "and that's why you should try to see it, despite a tremendously weak release platform!" Which is, by the way, something I think you should do. But as I am an honest man, I can't tell you to do it blind, and thus I am now, finally, going to share something that Kuenne took great pains to make sure nobody would even dream of before going into the film for the first time so caveat emptor, and are you really sure you want to keep reading? Okay then. Here goes: one month after Zachary's first birthday, Turner (who was out on bond, according to the wishes of the very nice Canadian judge lady) snuck away with her son, who was then legally in her possession, despite his obvious preference for Kate, tied him to her body, and stepped into the ocean, drowning herself and her infant son, leaving three older children by two previous marriages without a mother, and leaving Kate and David Bagby in the unthinkable situation of having predeceased both a child and a grandchild.

Right there was the moment that Dear Zachary went from being a fascinating true-crime piece to a Greek tragedy, and it's the nastiest shock I've ever received from a film. Which is hardly comparable to the infinitely nastier shock that the Bagbys got, which I'm certain is why Kuenne structured it that way. I feel awful about giving it away, but I could never recommend it otherwise: the abstract of dead children is one thing, but a very specific real child who died is something altogether else, and as someone who started bawling loudly when this happened, I can only imagine how gutted-out somebody might feel who actually likes children.

Having given this away does not, I hope, detract from the film's vast achievements: if only for using the step-by-step construction of a documentary as a metaphor for moving through the stages of grieving, it would be one of the most formally interesting non-fiction films of the year, and if only for the way that Kuenne ignores any pretense to objectivity, it would be one of the most emotionally rich. It is as deeply personal as any film you will ever see, and for that reason it is a profound experience; and what do we need the cinema for, if not to provide profound experiences? And for all that, because of all that, I am quite certain that I am never going to watch it again.

(By common consent, Tim, Carrie, and Rob have agreed to refrain from providing a star rating for this film. It seems crude to even consider such a thing).