Since childhood, 29-year-old Sato estimates he has spent “well over 10 million yen [US$68,000], if not 20 million” amassing merchandise and criss-crossing Japan to attend One Piece events.

So the office worker’s friends weren’t surprised when he and his bride, Junna, had a pirate-ship cake at their wedding in July and posed for photos beside a giant poster of Luffy and his gang in formal attire.

“They told me, ‘Shohei, it was so you,’” says Sato at his home in Tokyo, where plush toys of the reindeer-like character Chopper sit next to bookshelves packed with One Piece volumes. “I’ve lived my whole life alongside One Piece, so I wanted my wedding to honour it.”

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The latest movie in the franchise, One Piece Film: Red, was released in August and is already Japan’s highest-grossing film this year. Sato has seen it 21 times.

The film has also been a hit abroad, especially in France, while fans came out in costume for a huge Times Square advertising campaign ahead of the US release in November.

Inspiring dialogue, clever foreshadowing and relatable characters – author Eiichiro Oda is said to shed tears as he draws them – are often cited by One Piece super fans as the reason for their infatuation.

The manga’s plots are so intricate that publisher Shueisha holds a yearly quiz in which tens of thousands of fans compete to become its “knowledge king”. Sato once came in 10th, winning a golden trophy.

Another regular contestant, whose record is 15th place, is a systems engineer who goes by the online nickname Arimo.

Every night, after tucking his son into bed, the 32-year-old reads One Piece in his study, which is decorated with illustrations from the series. Even the walls of Arimo’s toilet are plastered with pages from the manga, and the geography lover has crafted his own globe to map the islands and oceans explored by Luffy.

“The One Piece world is so meticulously thought-out, I sometimes feel like there’s truly an alternative universe like this somewhere,” Arimo said.

One Piece is serialised in the weekly Shonen Jump magazine, which is aimed at teenage boys.

But its exaggerated humour, adrenalised action scenes and hundreds of varied characters appeal to a far broader readership. Natsumi Takezawa, 34, says she had “forgotten One Piece is supposed to be a boy’s manga”, because it “strikes a chord with all generations”.

She works part-time and reads One Piece for a little relaxation after picking up her five-year-old daughter from nursery, making dinner, bathing and playing with her, before finally putting her to bed.

“Without One Piece, I might feel drained,” she says. “I might be too exhausted by chores to do anything but sleep. That’s all my life could’ve been about. One Piece is my energy.”

Takezawa says the manga has even helped her with grief. Seeing the bawling Luffy realise “I still have my friends” after the death of his brother resonated deeply after she lost a close friend this year.

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“What kind of experience does a person have to go through to be able to draw such a powerful scene?” she says.

Sato thinks “another seven to eight years” would be a reasonable bet, however.

“One Piece is part of my life now, so I definitely want to see it through to the end,” he says.

“Until then, I refuse to die.”