The unidentified buyer made the winning bid Thursday for the card, handwritten by author J.K. Rowling as a charity fund-raiser. The money will be used to buy more than 18,000 books for schools in Africa.
Whether the buyer, who paid $45,314 for the card, chooses to keep the contents secret remains to be seen.
A Harry Potter Web site operated by the New York-based Leaky, Inc., had hoped to raise enough money to make the winning bid and possibly post the plot hints on the Internet.
The Web site raised about $24,000 for its bid. Editor in Chief B.K. DeLong called on the auction winner to share what is written on the card.
"While we've raised an incredible amount of money in such a short time for charity, we sincerely hope the winner of the card will be as charitable and share the 93-word summary with the thousands of fans who emptied their pockets for a chance at seeing it," he said in a statement.
The story of "Harry Potter and the Order of the Phoenix" is a closely guarded secret, and Sotheby's auction house in London only revealed that the words "Ron ... broom ... sacked ... house-elf ... new ... teacher ... dies ... sorry" are among the 93 words on the card.
Harry Potter fans, who have waited nearly 2½ years for book five, have grown increasingly curious about the fate of their hero as the author has declined to reveal a release date for "The Order of the Phoenix."
The first Harry Potter book, published in 1997, was "Harry Potter and the Philosopher's Stone" — released in the United States as "Harry Potter and the Sorcerer's Stone." The second and third volumes followed over the next two years, and the fourth, the mammoth "Harry Potter and the Goblet of Fire," appeared in July 2000.
Rowling produced the teaser for her next book at a Society of Authors conference earlier this year when several writers were asked to submit items for auction to help benefit Book Aid International. The charity supports book distribution in 40 of the world's poorest countries.
"It's a wonderful Christmas present for readers in some of the world's poorest countries," Book Aid International director Sara Harrity said.
The money from the sale will be used to buy 18,500 books for schools in Eritrea and elsewhere in Africa, Book Aid said.
"This is a really special gift for readers in Eritrea," said Gebrenegus Berhane of the Eritrean Ministry of Education in an e-mail to Press Association, the British national news agency. "A good book can light up a child's world."
By Audrey Woods
By Audrey Woods