When Tomoaki Hamatsu, an aspiring comedian from the Japanese province of Fukushima, auditioned for the reality show Susunu! Denpa Shōnen in January 1998, he arrived with zero expectations and a dream for some fame. Reality television was still in its wild west infancy – no contracts, no protections, still just a handful of home-grown personalities. The new frontier of reality celebrity was just opening up, and Hamatsu, who went by the childhood nickname Nasubi – the Japanese word for eggplant, owing to his long face – saw an opportunity. His only boundary was a request from his mother: “Don’t get naked.”

As captured by the show’s producers and now reassembled in the remarkable new Hulu documentary The Contestant, a young Nasubi – short hair, wide and near-blinding smile – “won” the audition with the right lottery ticket. His prize? A stranger-than-fiction 15-month ordeal and groundbreaking national celebrity. Not that Nasubi was aware of it, at the time; he was escorted straight from the audition, blindfolded, to a studio apartment in Tokyo furnished with nothing but a camera, a table, a radio, a phone, a cushion and a full magazine rack. Ordered to strip naked, Nasubi tentatively undressed, still hamming for the camera. His assignment was to obtain everything he needed (clothes, food, entertainment) via magazine sweepstakes contests, until he won the equivalent of one million yen ($8,000).

So began A Life in Prizes, Denpa Shōnen’s massively popular segment, 17 million viewers a week at its peak, starring an unwitting Nasubi, who claims in The Contestant that he was told none of the footage would go to air. “‘If you do this reality show, it will be this way’ – no, there was nothing, no precedent,” Nasubi, now 48, told the Guardian via a translator. “The TV stuff, I didn’t even think about that. I didn’t know. I had no idea where it was going.”

Neither, it seems, did the producers, who made up the game and changed the rules at their whims, and their viewers. Denpa Shōnen was the ever-evolving brainchild of the famed producer Toshio Tsuchiya, pitting young people in survival or prank situations accentuated with cartoonish graphics and cheery hosts. The through-line, as Tsuchiya explains in the film, was that “if you drop anyone into any situation, give them a task and then capture it on camera, actually all humans are entertaining”. A Life in Prizes felt like a stroke of TV genius. “At the time, I wanted to capture something amazing, something incredible,” says Tsuchiya. “An aspect of humanity that only I, only this show, could capture.”

As depicted in The Contestant, the line between “reality show production” and “human experiment” was vanishingly thin. The documentary, directed by Clair Titley, revisits the saga from Nasubi’s perspective. The tone of A Life in Prizes was jaunty, comical and irreverent; in archival footage redubbed with English graphics and narrated, in direct translation, by Fred Armisen, Nasubi is a pitiable but endearing presence, an easy character to root for as he dances with every much-needed win and stretches his face in exaggerated glee. (Naked throughout the whole series – he never won clothes he could wear – producers covered his genitals with a large eggplant animation; Juliet Hindell, a BBC correspondent in Tokyo at the time, speculates that this led to the emoji’s association with the penis.)

“I really wanted a western audience to understand what it was like, as a Japanese person, to watch that,” said Titley of the archive’s translation to English. “How intense and insane it was, how they were trying to make it funny.” Even moments that read as torture – Nasubi resorting to eating dog food to survive, a too-scrawny Nasubi winning rice but not a means to cook it – were played for laughs. The Contestant toggles between this aired footage and Nasubi’s recollections of his private loneliness and despair, including, as the months wore on, suicidal thoughts. “We did quite a lot of manipulation on the archive, but with the intention of making it more honest,” said Titley, “so you could get this sense of this man naked, alone in a room.”

Alone, yet watched – some of Nasubi’s strange story feels right out of The Truman Show, also released in 1998. His journals, the only outlet for his private thoughts during his 15 months indoors, were published without his knowledge and became bestsellers. Responding to audience enthusiasm, Denpa Shōnen began a 24/7 livestream of his room, with producers using a joystick to keep the eggplant in real-time place. To keep things fresh when Nasubi finally reached his goal, Tsuchiya surprised him with a trip to Korea … and another Life in Prizes challenge, this time in Korean, a language Nasubi could not speak or read. The reveal of his celebrity is too surreal to spoil.

And yet, Nasubi kept going, refusing to quit the show. Though Nasubi describes a psychological state akin to Stockholm syndrome in The Contestant and a 2014 This American Life segment, he and the film decline to call it as such. Still, at the end of his time on Denpa Shōnen, “there was a big hole in my heart,” he said, “because humans could be so cruel just in order to make in an interesting, popular program. To get the popularity. It looks like a comedy, but people do not understand the hardship and all the struggle that I went through within. That wasn’t really shown.”

Seeing the footage now is “like revisiting the hard times. It’s very difficult,” he said. “But when you look objectively, I could see: ‘OK, somebody is having a hard time, living really, really as best as he possibly can, in a limited situation.’ That kind of human spirit, you feel it.”

Given Nasubi’s experience in the ruthless early days of reality television, “consent was a huge part of this film,” said Titley. “We talked a lot. I told him what we were doing and why we were doing it. I asked for his input on certain things, visual ideas. We talked through the reasons behind everything and checked in with him.”

The final third of the film extends beyond A Life in Prizes, as Nasubi found renewed faith in humanity through helping rebuild Fukushima after the devastating earthquake and tsunami, eventually summiting Mt Everest and even collaborating with Tsuchiya on raising and awareness and funds. “In order to accept what I went through in the past – that became a journey. I had to get over that,” said Nasubi. “The human can change, that’s one thing that I really realized. We cannot stay obsessed with one thing and let your spirit die. I don’t want to do that. I want to be open and accept everything that passed.”

That includes not watching the many reality television series spawned in the wake of Denpa Shōnen, from Survivor to Big Brother to Japan’s Terrace House, to name just a few points in what is now a vast content universe still reckoning with its effects on participants. Still, “there’s no reality show [contestant] that actually went through what I went through,” said Nasubi. The Contestant, with its cool and narrow approach to this bizarre chapter in TV history, makes as good a case as any for keeping it that way.

• The Contestant is now available on Hulu in the US with UK and Australia dates to be announced