★★★½

It would be unfair to spoil the surprises in David Farrier and Dylan Reeve's investigative documentary Tickled, but the premise should be enough to draw you in. Farrier, the film's narrator, is a New Zealand journalist specialising in light entertainment and general weirdness. Over the opening credits, there are clips of him interviewing everyone from a local donkey fancier to Justin Bieber.

When Farrier came upon a series of online videos depicting "competitive endurance tickling", this seemed a natural subject for his next story. But his request to speak to the producers was met with a string of emails loaded with homophobic abuse, describing this supposed sport as a "passionately and exclusively heterosexual endurance activity".

At this point the quest became personal, and also a matter of national pride with underdog New Zealanders pitting themselves against bullying Americans. Farrier and Reeve headed for the US to find the truth behind the videos, armed with hidden cameras and a polite but steadfast manner. What they uncovered was considerably more sinister than the whimsical human interest story Farrier originally had in mind.

The material is sufficiently bizarre that the filmmakers are wise to avoid too much zaniness on their part. Farrier plays the bumbling everyman in the tradition of Louis Theroux (whom he physically resembles) or Nick Broomfield (who more or less invented the documentary style). But his humour is subdued and he doesn't overplay the role of the "regular guy" plunged into a nest of weirdos. When he interviews avowed tickling fetishist Richard Ivey – who turns out to be one of the film's producers – he's bewildered and somewhat repulsed, but also curious and respectful.

Tickled is not really a film about the psychology of fetishism. The character who emerges as the villain appears only briefly on camera, and remains a shadowy figure to the end. The focus is on Farrier and Reeve's repeatedly frustrated efforts to get to the heart of the mystery. The final revelations emerge through a lucky discovery that any screenwriter would reject as too contrived.

The film may be complete, but the story of Tickled is still in progress. To read the latest updates you'll need to look on Google (it's only fair to mention that for the moment, no fresh criminal charges have been laid). Even with questions left unresolved, this is a great yarn – one which, like tickling itself, induces both pleasure and discomfort.