HAIM‘s Thursday concert in London featured a surprise guest: the one and only Taylor Swift.

The pop superstar joined the sister trio to perform a mashup of her hit “Love Story” and the HAIM track “Gasoline.”

“I heard that my girls were playing in London at the O2 and I thought, ‘I’m gonna have to see them!'” Swift told the cheering crowd in video shot by Twitter user @RyanJayBailey. “And it looks like there’s about 20,000 other people who also thought that.”

“So, we had a thought,” she continued. “That if we were to do some sort of mashup, we can possibly, maybe, get you to sing the loudest that you’ve ever sung all night.”

The original “Gasoline” appears on HAIM’s latest album, 2020’s Women in Music Pt. III. Swift guests on a remixed version of the song, which is included in the 2021 deluxe edition of Women in Music Pt. III.

The London concert was the last date on HAIM’s U.K. tour. The group is set to open the first show of the Red Hot Chili Peppers‘ U.S. tour, which kicks off Saturday in Denver.

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