For many people, I suspect, tickling is a verb, not a world. You would be wrong! Tickling has more going for it than a red plush toy named Elmo — it has scholars, scientists, deep thinkers. In his multivolume classic “Studies in the Psychology of Sex,” the psychologist and pioneering sexpert Havelock Ellis dedicates an entire section to tickling, defining it as both “a specialized modification of touch” and “the most intellectual mode of touch sensation.” Everyone from Aristotle to Darwin and Freud had something to say about the teasing art of the tickle, as did the Marquis de Sade.  There’s even more about its ecstasies and agonies in “Tickled,” a terrifically entertaining documentary about a strange, murky corner of the adult tickling world. A New Zealand journalist named David Farrier discovered this touchy, feely realm when he came across an online video. During his career, Mr. Farrier has interviewed Justin Bieber, reported on eels and looked into the cover art of black-metal bands. He has a particular talent for the outré, however, having made, as he puts it in the movie, “a career out of looking at the weird and bizarre side of life.” So when he came across a video of cute, buff and clothed young men tickling and being tickled, he knew that he had a story.  The Times critic Manohla Dargis reviews “Tickled.”  What he learned was that an American company, Jane O’Brien Media, was holding a monthly event called Competitive Endurance Tickling. The contest was open to young athletes, who, if chosen, would receive free airfare to the United States, hotel accommodations and 1,500 bucks. Mr. Farrier — who has an enviable deadpan — says that he wondered if this was a “tickling league,” noting that participants were all in Adidas gear. He doesn’t say anything about the ticklees in wrist and ankle restraints; if this were a league it was the kind the characters in “Fifty Shades of Grey” might join. Instead, like the reporter he is, Mr. Farrier turned his inquisitiveness into an investigation.  For starters, he contacted Jane O’Brien Media, which led him into a curiouser and curiouser adventure, starting with a company representative, Debbie Kuhn, who was very unwelcoming. “To be brutally frank, association with a homosexual journalist is not something” the company would embrace, a message from Ms. Kuhn read. “We desperately do not want a homosexual participant base applying for this project.” This only further intrigued Mr. Farrier, who is gay, and he continued to reach out to Jane O’Brien Media and Ms. Kuhn, who, in turn, insisted that this was a “passionately and exclusively heterosexual athletic endurance activity.” Ms. Kuhn also began lobbing homophobic slurs.  A preview of the film.  The epithets, the secrecy, the musclemen, the tickling — taken together, it was journalistic catnip. Mr. Farrier decided to make a documentary with a co-director, Dylan Reeve, an undertaking that grew rapidly surreal, soon involved legal threats and put the filmmakers on a plane to North America. There, in a bid to learn more, they chased leads and ventured deeper into the world of tickling, meeting one charming enthusiast who had turned a personal passion into a profession with a camera, some choice tools of the trade (an electric toothbrush) and a light ambidextrous touch. They also met other journalists who had reported on a tickling mystery man whom some fear and loathe.  To say more would lessen the impact of Mr. Farrier and Mr. Reeve’s discoveries, which they piece together with restraint and fine technique, using a clear, direct approach that serves the material. (At this point, I should mention that a defamation lawsuit was filed on behalf of the mystery man.) They more or less stick to the documentary-as-investigation model, in which a filmmaker plays the onscreen investigator-inquisitor. Mr. Farrier, however, is less of a showboat than some documentarians who assume that role, and, after a while, this measured quality feels as much an ethical choice as a matter of temperament. He and Mr. Reeve see the humor, but they also see the pathos — because it’s all fun and giggles until someone gets hurt.  “Tickled” is rated R (Under 17 requires accompanying parent or adult guardian). For language, not tickling. Running time: 1 hour 32 minutes.  Directors David Farrier, Dylan Reeve  Stars David Farrier, Dylan Reeve  A version of this review appears in print on June 17, 2016, on page C8 of the New York edition with the headline: A Feather-Light Touch, Until It Isn’t Anymore. Order Reprints| Today's Paper|Subscribe
Two filmmakers had barely begun their reporting for this documentary when threats from the private world of tickling began.