Taylor Swift joins HAIM on stage in a surprise appearance at London Concert

Taylor Swift made fans' wildest dreams come true when she made a surprise appearance during HAIM's sold-out July 21 show in London—and lucky for viewers, plenty of footage has surfaced on the Internet to relive this moment forever.

HAIM was rocking out to their hit song "Gasoline" (which features Taylor), but halfway through the track, the group stopped to welcome the "Bad Blood" singer on to the stage. Taylor and the HAIM sisters then delivered an edgy mash-up performance of "Gasoline" and Taylor's iconic solo song "Love Story."

While onstage, Taylor said that their idea to collaborate during the concert was all part of a master plan to get the crowd "to sing the loudest you have sung all night."

She added, "Which is a big challenge because you have been singing very loudly—extraordinary work."

Taylor noted that it's been a while since she has performed in concert—while also shouting out HAIM's sold-out status at the O2 Arena.

"I haven't been on stage in a very long time," she said. "It's nice…it's nice, it's very nice [to be back]. When I heard my girls were playing in London at the O2, I thought, 'I'm going to have to see that.' And it looks like there's about 20,000 other people who also thought that."