David Farrier had built a career on the strange. As an entertainment reporter in New Zealand, he became the country’s go-to source for weird and wacky entertainment.

Then a friend sent him a peculiar video, and his life became far stranger than he ever expected.

The video? It was of something called “competitive endurance tickling.” Participants were tied down and tickled for as long as they could stand. The Facebook page that posted the videos, Jane O’Brien Media, had more than 20,000 followers.

“Just from the three words ‘competitive endurance tickling,’ I was like, ‘This is incredible. This is going to be the best three-minute news story of my career,’ ” Farrier recalled.

It became something much bigger. Specifically, a full-length documentary titled “Tickled,” which premiered Sunday night at the Sundance Film Festival. The perplexing, engrossing doc isn’t really about the world of tickling fetishes generally. Instead, Farrier and co-director Dylan Reeve try to figure out who specifically is behind these competitive endurance tickling videos. Figuring that out becomes a monumental task — the mastermind’s identity seems to be the world’s best-kept secret.

That the videos exist was rather strange, sure, but stranger things exist. What about this merits an entire documentary? The real kicker was Jane O’Brien Media’s response to Farrier’s inquiry. The company responded with vicious personal attacks on Farrier and his sexual orientation, and threatened extreme legal action should he inquire further. Undoubtedly a strange reaction from a company with that kind of social media following.

“The response took it up a notch so quickly,” Farrier said. “I think when the private investigator showed up at my doorstep, and when we started receiving letters from lawyers telling us to stop what we were doing … when they started hiring lawyers telling us we couldn’t blog about them anymore, for me, that was when it hit.”

Few things came easy for Farrier and Reeve. They reached out to tons of people impacted by these videos. Only two of the video participants agreed to be interviewed on the record.

“It wasn’t because they were embarrassed or ashamed,” Farrier said. “It was that they didn’t want to provoke another response.”

Though Jane O’Brien Media told all the video participants their real names wouldn’t be used, they began breaking that promise. Participants who confronted the company became subject to vicious, damaging, life-altering smear campaigns.

According to Reeve, participants shared some commonalities. For one, they were all male, young and athletic. In addition, most were financially desperate, and Jane O’Brien Media offered them large sums of money for their participation. That the videos have homoerotic undertones, and the participants consider themselves athletes, makes them the perfect blackmail target.

“When you Google your name, and every result is you being tickled, that becomes problematic,” Farrier said.

“If you’re being harassed online, who do you tell? The Internet police? Where are they?,” Reeve added. “Anywhere where there was any kind of pushback against the control or power, that’s when it seemed to go a little off the rails.”

As “Tickled” goes further down the rabbit hole, and its directors traverse the United States to find the truth, it becomes clear that tickling is only part of a larger issue. It isn’t just about tickling, but control.

“It made a lot of the behavior toward us make more sense,” Farrier explained. “When you suddenly see how having control over someone, and being able to tie them down, links with this whole thing of them terrifying us with legal threats. When you see that whole world come together, it was a bit of a revelation, really.”

Understandably, that kind of manipulation took a toll on Farrier and Reeve. Because the duo had to dig through so much secrecy and manipulation, Reeve said, their theories and instincts were constantly being upended. It became difficult to trust anyone they contacted.

“I think had either of us been doing it on our own, it might not have happened,” Reeve said. “I think the peril that we felt might have been enough to say, ‘The cost-benefit here doesn’t stack up.’ “

“Tickled” is probably unlike any documentary you’ve ever seen. As its many mysteries are solved, it’s clear that sometimes fact really is stranger than fiction.

For more reviews, and ongoing coverage of Sundance 2016, head to heraldextra.com/sundance.