After Kate Middleton and Prince William made their marriage official at Westminster Abbey on April 29, 2011, they had the privilege of attending two separate wedding receptions hosted by the late Queen Elizabeth II and King Charles III. At the second reception, 300 close family and friends gathered to celebrate the newlywed couple in the Throne Room at Buckingham Palace – and according to royal biographer Robert Jobson’s new book, Catherine, Princess of Wales: The BiographyThe duo had a few surprises in store. One of them made a particularly big impression on the guests.

In his biography of the Princess of Wales’ life, Jobson says that after Ellie Goulding had performed several of her hits for the newlyweds, Kate and William put on a performance of their own. “They stood in the middle of the dance floor holding hands and grinning,” he writes. “Then suddenly the first bars of the song ‘You’re the One That I Want’ from the musical Fat rang out loudly. William and Catherine then began dancing around, pointing at each other and singing along to the words in the style of the main characters Danny and Sandy. The house was hilarious.”

While the couple’s surprise show was definitely a standout moment, there were many other aspects of the soirée that made it “the most magical party you could imagine,” as one guest put it, according to Jobson’s book. Kicking off the evening’s entertainment, Goulding belted out a cover of Elton John’s “Your Song” during Kate and William’s first dance – an experience the artist described as “terrifying.” She then sang “Starry Eyed” and her rendition of The Killers’ “Mr. Brightside,” one of the groomsmen’s favorite songs, according to Jobson. “William and Kate were in the thick of it and did their best,” a source said People back then. “They really put in the effort. They were in a full party mood and stayed until the last minute.”

After the newlyweds channeled their inner Sandy and Danny, guests headed to the lawn behind the palace for another surprise: a fireworks display. According to Peoplewhile the crew gathered outside, Kate and William sat on the roof of a balloon-decorated Fiat while a servant drove them away (though they did not leave the grounds). Then the show began, which an insider says was grand but quick – it reportedly lasted just two minutes. By 3 a.m., the party had died down, and “the newlywed couple were the only people in the palace,” says Jobson.

In addition to giving readers a deeper look into Kate and William’s wedding, Jobson also revealed other unknown details about their relationship. Before the couple got engaged, shortly after they graduated from college at St. Andrews, William reportedly broke up with Kate because he wasn’t ready for an engagement. After the Prince of Wales canceled his New Year’s Eve plans with his girlfriend, he called her to break up, explaining that they needed “a bit of space” to “find our own path,” according to Jobson. “In an emotionally charged 30-minute conversation, both admitted they had ‘different views,'” Jobson says in his book. “It was a devastating blow to Catherine, who felt doubly disappointed at being dumped on the phone.” Eventually William realised he had made a mistake and, after meeting Kate again at a fancy dress party, they moved in together and became engaged.