It seemed almost too good to be true, and in the end it was. A conscientious pianist who had enjoyed an active if undistinguished career in London falls ill and retreats to a small town. Here she undertakes a project to record virtually the entire standard classical repertoire. Her recordings, CDs made when she was in her late 60s and 70s, are staggering, showing a masterful technique, a preternatural ability to adapt to different styles and a depth of musical insight hardly seen elsewhere.

Born in 1928, the pianist, Joyce Hatto, was the daughter of a music-loving London antiques dealer. As a teenager, she said, she kept practicing during the Blitz, hiding under the piano when the bombs were falling. She claimed later to have known the composers Ralph Vaughan Williams, Benjamin Britten and Carl Orff, to have studied Chopin with the French virtuoso Alfred Cortot and taken advice from the pianist Clara Haskil. She was Arnold Bax's favored interpreter for his Symphonic Variations.

Ms. Hatto made recordings from the 1950s until 1970 — some Mozart and Rachmaninoff — but tending toward light-music potboilers: Hubert Bath's Cornish Rhapsody and Richard Addinsell's Warsaw Concerto. Her career was already in decline when she was given a cancer diagnosis in the early 1970s. She retired to a village near Cambridge with her husband, a recording engineer named William Barrington-Coupe, and a fine old Steinway that Rachmaninoff himself had used for prewar recitals in Britain.

Then came one of the strangest turns in the history of classical music. Starting in 1989, Joyce Hatto began recording CDs for a small record label run by her husband. he began with Liszt, went back to cover Bach and all of the Mozart sonatas and continued with a complete Beethoven sonata set. Then on to Schubert and Schumann, Chopin and more Liszt. She played Messiaen. Her Prokofiev sonatas (all nine) were tossed off with incredible virtuosity. In total she recorded more than 120 CDs — including many of the most difficult piano pieces ever written, played with breathtaking speed and accuracy.

Intriguingly, she gave to the music a developed although oddly malleable personality. She could do Schubert in one style, and then Prokofiev almost as though she was a new person playing a different piano — an astonishing, chameleon-like artistic ability.

We normally think of prodigies as children who exhibit some kind of miraculous ability in music. Joyce Hatto became something unheard of in the annals of classical music: a prodigy of old age — the very latest of late bloomers, "the greatest living pianist that almost no one has heard of," as the critic Richard Dyer put it for himself and many other piano aficionados inThe Boston Globe.

Little wonder that when she at last succumbed to her cancer last year at age 77 — recording Beethoven's Sonata No. 26, Les Adieux, from a wheelchair in her last days — The Guardian called her "one of the greatest pianists Britain has ever produced." Nice touch, that, playing Beethoven's farewell sonata from a wheelchair. It went along with her image in the press as an indomitable spirit with a charming personality — always ready with a quote from Shakespeare, Arthur Rubinstein or Muhammad Ali. She also had a clear vision of the mission of musical interpreters, telling The Boston Globe: "Our job is to communicate the spiritual content of life as it is presented in the music. Nothing belongs to us; all you can do is pass it along."

Now it has become brutally clear that "passing along" is exactly what she was up to. Earlier this month, a reader of the British music magazine Gramophone told one of its critics, Jed Distler, that something odd happened when he slid Ms. Hatto's CD of Liszt's Transcendental Études into his computer. His iTunes library, linked to a catalogue of about four million CDs, immediately identified it as a recording by the Hungarian pianist Laszlo Simon. Mr. Distler then listened to both recordings, and found them identical.

Since then, analysis by professional sound engineers and piano enthusiasts across the globe has pushed toward the same conclusion: the entire Joyce Hatto oeuvre recorded after 1989 appears to be stolen from the CDs of other pianists. It is a scandal unparalleled in the annals of classical music.

Ms. Hatto usually stole from younger artists who were not household names, although on the basis of the reviews she received, they richly deserved to be. Her recording of Chopin mazurkas seems to be by Eugen Indjic; the fiendishly difficult transcription of Chopin studies by Leopold Godowsky are actually recordings by Carlo Grante and Marc-André Hamelin; her Messiaen recordings were by Paul S. Kim; her version of the Goldberg Variations of Bach at least in part by Pi-Hsien Chen; the complete Ravel piano music by Roger Muraro. As reports come in, the rip-off list grows daily.

Her concerto recordings are even more brazen. The CD labels say they were made with the National Philharmonic Symphony Orchestra, always conducted by one René Köhler. Mr. Barrington-Coupe told a reporter that this was his name for a pick-up orchestra of Polish émigrés who, he said, came out from London to record at a venue he now refuses to reveal. He declined to further discuss the orchestra on the grounds that they were employed "below union rates." No one has yet been able to find a single reference to this René Köhler outside of the Joyce Hatto recordings, nor have any members of the orchestra come forward to confirm Mr. Barrington-Coupe's story.

In a rapturous review of Ms. Hatto's playing of Rachmaninoff's Third Concerto, one critic said of the orchestra musicians: "It doesn't matter who they are, their playing is tight and hot." Actually, it did matter, since they have turned out to be the Philharmonia Orchestra of London, conducted by Esa-Pekka Salonen, performing with the formidable Yefim Bronfman. Her version of the Brahms Second Concerto is Vladimir Ashkenazy's, with the Vienna Philharmonic under Bernard Haitink laboring in the name of René Köhler and his non-union Poles.

Since the news broke, some have likened the exploits of Joyce Hatto to the notorious 20th-century Vermeer forger Han van Meegeren. But the differences are significant. Van Meegeren's success was based as much on presentation — stories of old Italian families impoverished before World War II and needing quick cash — as on artistic plausibility. After he confessed, it was not hard for anyone to see that his dreadful fakes had more in common with each other than with any original Vermeers.

Joyce Hatto, however, was not a pianistic forger. In order to forge a piano performance, she would have had to record Beethoven's Hammerklavier herself and sell it to the world as a lost recording by, say, William Kapell. She was instead a plagiarist: she stole other pianists' work and, with only a few electronic alterations, sold it as her own.

Although the critics who praised Van Meegerens's "Vermeers" as masterpieces were in the end rightly humiliated, the same should not be true of those who praised Ms. Hatto's recordings. They may have been fooled, but their opinions were not foolish, because the artists she ripped off played beautifully.

Yet the Joyce Hatto episode is a stern reminder of the importance of framing and background in criticism. Music isn't just about sound; it is about achievement in a larger human sense. If you think an interpretation is by a 74-year-old pianist at the end of her life, it won't sound quite the same to you as if you think it's by a 24-year-old piano-competition winner who is just starting out. Beyond all the pretty notes, we want creative engagement and communication from music, we want music to be a bridge to another personality. Otherwise, we might as well feed Chopin scores into a computer.

This makes instrumental criticism a tricky business. I'm personally convinced that there is an authentic, objective maturity that I can hear in the later recordings of Rubinstein. This special quality of his is actually in the music, and is not just subjectively derived from seeing the wrinkles in the old man's face. But the Joyce Hatto episode shows that our expectations, our knowledge of a back story, can subtly, or perhaps even crudely, affect our aesthetic response.

The greatest lesson for us all ought to be, however, that there are more fine young pianists out there than most of us realize. If it wasn't Joyce Hatto, then who did perform those dazzlingly powerful Prokofiev sonatas? Having been so moved by hearing "her" Schubert on the radio, I've vowed to honor the real pianist by ordering the proper CD, as soon as I find out who it is. Backhanded credit to Joyce Hatto for having introduced us to some fine new talent.

[First published as an OpEd piece in The New York Times, February 26, 2007.]