Opposite Gruff Rhys, in the diminutive whitewashed cottage where we meet in his home town of Bethesda, north Wales, sits his companion John Evans. John, with his big Sesame Street eyes, stares blankly at Gruff and doesn’t move.

John’s a puppet. He’s three feet high and made of felt. He is decked out in breeches, a ruffled white shirt and his feet lie motionless in black buckled shoes on the edge of the sofa. Gruff prefers the description “avatar”. He tells us how John the avatar “got mobbed” on tour as a target for “selfies” and drew talk of voodoo powers. And so, effortlessly, we enter the intriguing, idiosyncratic world of indie rock musician Gruff Rhys.

I hope I haven’t completely ruined John Evans’s name by turning him into a 3ft felt avatar. Gruff Rhys, singer

Avatar John was made specifically for Rhys’s latest project, American Interior, an eccentric and engaging take on a little-known figure in Welsh history called John Evans. With his group Super Furry Animals “in hibernation”, as he puts it, this is another solo departure for Rhys.

But it is perhaps his most critically acclaimed yet. For which his country might yet owe him a small debt of gratitude. As together with his 3ft high friend, this “lovably odd” musician – as he’s been aptly described – may just have helped Wales rediscover one of its most intrepid sons.

“I hope I haven’t completely ruined John Evans’s name,” says Rhys, “by turning him into a 3ft felt avatar and simplifying his story too much, but I think it’s a story worth celebrating”. His gentle, slow (very slow at times) delivery and understated humour is punctuated sporadically by a deep smile. The story he tells, meanwhile, is as obscure as it is surprising.

John Evans, a far-flung relative of Rhys, we learn, left his home in Waunfawr, on the edge of Snowdonia, in 1792 having been “radicalised” by a local poet. Amidst the revolutionary fervour of the period, a fantastical story about a tribe of Welsh-speaking American Indians had gained currency. Madoc, a Welsh prince, had set foot in America in 1170 and left behind a “lost tribe” of Welsh speakers.

So the legend went. If Evans could prove the link and find the tribe, then he could lead an exodus of Welsh radicals away from his royalist homeland to found a new state in the New World. This is the bizarre odyssey which Rhys documents in American Interior.

The film, directed by Dylan Goch, follows Rhys around America. By day he delves deeper into the world of the 22-year-old John Evans and by night he takes his research onto the stage in a series of solo gigs. The result is a blend of PowerPoint presentation and acoustic guitar performance, at one point featuring an old friend of the singer who was initially bemused by the project.

“He [Gruff] told me the story of John Evans,” says ex-Flaming Lips drummer Kliph Scurlock (who plays on the album accompanying the film), “and I thought he was f***ing with me!”

Rhys says he was trying to invent a tour which had a sense of purpose: “I called it an investigative concert tour, so at night I was playing concerts and during the day I was like Colombo… I was into the idea of making an album that people can get into for months, completely immerse themselves in and come out with a pretty unexpected knowledge of this guy John Evans, his journey, maybe some of the history of American Europe.”

Rhys’s detective work leads him eventually, as it would a malaria-stricken John Evans, to North Dakota and the Mandan nation, supposedly the land of the “lost tribe”. Here the singer, whose first language is Welsh, meets the last fluent Mandan speaker, prompting what he describes as a “very profound” encounter:

“I often take my first language, Welsh, for granted, and it’s made me realise how precious the language is and what an opportunity we have to save what is also a language at crisis point.”

He is passionate about the subject. Salvaging the memory of John Evans – the adventurer from Waunfawr – might also appear laced with a hankering for an older Wales, but reinvention rather than nostalgia seems to be the driving force here.

It would take John Evans four long and extremely arduous years to reach the Mandan. In the process, according to Rhys’s book, “he wrestled the largest river reptiles ever seen in the Mississippi, hunted bison with the Omaha tribe, defected to the Spanish in St Louis, annexed North Dakota from the British and created the map that guided Lewis and Clark on their legendary expedition.”

Ultimately, however, it ended in failure. There was no tribe of Welsh speakers. And Evans would never make it home. He died in New Orleans, aged 29.

But there was a legacy, insists Rhys: “Similar to most people’s lives, you have ambitions to maybe achieve a certain thing and you end up doing something completely different and it’s just as valid… for example, I had no ambition to be a guitarist and somehow I’ve ended up making a living going around the place playing the guitar and singing, which is ridiculous.”

American Interior is an epic, ultimately tragic tale of a young man deluded by myth. The invention and flair, however, with which this immensely likeable musician retells it, captures the imagination in truly life-affirming style.

The film is currently being shown in selected screenings around the country. It is due to receive its television premiere on S4C in the autumn.