The online world of “competitive endurance tickling,” in which tickle fetish videos are framed as a “sport,” is of course a deeply strange one.

But as far as weird things on the internet go, on the surface at least, it doesn’t rank very highly.

The documentary “Tickled” began when New Zealand journalist David Farrier launched a probe into this subculture and transformed into something entirely different when his first inquiries to production company Jane O’Brien Media resulted in a stream of threats and homophobic invective in return.

The less said about the ensuing revelations the better; it’s a journey into dark recesses of manipulation and harassment, with one hard-to-believe turn after another. The movie fascinates precisely because Farrier (and co-director Dylan Reeve) have a steadfast refusal to surrender when some unsettling, bullying behavior is directed their way. In unpacking the labyrnthine racket behind these relatively benign videos, they reveal something deep and fundamental about the importance of preserving journalism’s most fundamental mission: seeking the truth.