“It’s come pretty much out of nowhere,” says British Museum curator Tim Clark, placing a small wooden box on the table — it’s about the dimensions of a shoebox, slightly weathered and lightly inscribed with fluid kanji characters. “It was in Japan until last summer, where it belonged to a dealer, and before that, we don’t know. In fact there’s still a lot about it we don’t know.”

And with that, he takes out a compact bundle, loosens the silk cord around the worn cloth cover, and lays the Japanese section’s latest, almost half-a-million-pound (¥75 million), acquisition gently down on the table and starts unrolling it. I have my dictaphone running, and when I listen back there’s almost a minute when I’ve gone completely silent as I watch Clark reveal this treasure — which goes on display to the general public on April 18.

The piece is a jawdroppingly fine, 15-meter-long handscroll depicting the arrival in Japan of Commodore Matthew C. Perry and the nine famous black ships in February 1854. It was Perry’s second visit, and culminated in the signing of the Convention of Kanagawa on March 31, which effectively ended Japan’s centuries of sakoku (closed country) diplomatic seclusion.

The scroll opens, like many a Hollywood movie since, with a wide panorama. “There were two fiefs which were charged with the defence of Japan: Kokura and Matsushiro, so we begin with the panning shot of the defensive forces in all their glory,” explains Clark. “This is what’s going to be the treaty house where they do the negotiations; this is the local shrine, still completely undeveloped.”

Cinematically, this magnificent opener is succeeded by more focused vignettes. “We’re now zooming in from the wide-angled shot,” says Clark. “Here are Perry and (Commander Henry) Adams coming up the beach. It’s like Nixon coming down the stairs of the aircraft to greet Zhou Enlai.”

Clark’s scene-by-scene commentary, as he rolls the scroll up at one end and out at the other, is likely just how the scroll would have been used by its first owner. Notably, there is no explanatory text in the scroll itself, just an introductory preface. This suggests that the scroll’s owner was someone who needed no explanation — in other words, someone who was present at the events depicted, and would tell the story himself to the favored guests who were permitted to view the work. So who was that owner? And, indeed, who was the artist? These are, it turns out, two more of the things we don’t precisely know about this remarkable piece.

Since — and before — acquiring the scroll, Clark has been doing some sleuthing, with the assistance of Japanese scholars, in particular those of the Reihaku, the National Museum of Japanese History. We know who wrote the preface, an eminent poet of the Chinese style named Onuma Chinzan (1818-1891), “so the scroll’s owner was obviously moving in high literary circles in the city of Edo,” explains Clark. “Chinzan writes: ‘Mr Maruyama had an artist paint this.’ But he doesn’t” — Clark gives a laugh of gentle exasperation — “say who the artist is.”

Maruyama’s own identity is also vague — after all, the name is not uncommon. But one of Clark’s Japanese correspondents showed him a poetry diary entry for 1858 — the year of the scroll’s completion — in which Chinzan goes mountain climbing with a Mr. Maruyama. The diary locates the pair inside the Matsushiro fief — one of the two tasked with Japan’s national defence, as shown in the scroll’s opening scenes.

Here’s where the detective work steps up a gear: the Sanada family ruled Matsushiro, and Clark has been directed to an obscure 1930s journal article which reproduces sketches made by a mid-19th century artist retained by the Sanada that are near-identical to scenes in the British Museum’s scroll. The article (authored by the artist’s son) at last gives us a name: Hibata Oosuke (1813-1870). “We can’t be totally certain yet,” says Clark, “but everything triangulates.”

As the scroll unrolls to reveal further gorgeous — and surprisingly lively — scenes of banqueting, dancing, of amazed American sailors patting the bellies and squeezing the muscles of sumo wrestlers, it is hard to understand why Japan let such a treasure go, even though other pictorial versions of the event do exist in locations within and outside Japan. “For the British Museum, with its ambitions to tell the big picture in history,” says Clark, “it is almost like our Japan Galleries were set up waiting for something of this importance and great historical and artistic interest.”

From April 18, for six months, the scroll will be displayed at the center of the gallery, a few meters visible at a time — repeat visits will be necessary to savor the full magnificence of the piece. The theme of the surrounding gallery exhibition, “The Making of Modern Japan,” provides excellent context — there are, for example, lithographs that comprise the American record of Perry’s visit.

And herein lies the historical value of the scroll — for the insight it gives into Perry’s visits as viewed by the Japanese. We’re used to a narrative of shock and awe: the Americans arriving by steamship, Commodore Perry dropping not-so-subtle hints about the offensive capability of his shell guns. The scroll tells a very different story: American officers inspect the chinaware at the treaty banquet, sneak food out in their hats to share with those too junior to attend, have their hand wrung painfully by a sumo wrestler.

“It’s the kind of thing you don’t get in the American lithographs, where everything’s going like clockwork,” says Clark. “Throughout, you get this human detail. What attitude does that actually reveal toward to Americans? It doesn’t seem to see them as a threat, more a curiosity — these people who do things differently. This scroll gives us another side of the story.”

The Perry Scroll is on display in the Japan Galleries of the British Museum, London till Oct. 13. Free; Open daily, 10 a.m.-5:30 p.m. (Fridays till 8:30 p.m.). For more information visit www.britishmuseum.org