London, UK - Billie Eilish had an extra-special surprise in store for music fans in London as she brought out boygenius for a joint performance of when the party's over.

The 21-year-old hit the stage at London's Electric Ballroom on Tuesday for a small show amid her recent headlining festival appearances.

The indie supergroup, which consists of Julien Baker, Phoebe Bridgers, and Lucy Dacus, were welcomed on stage for a dreamy rendition of Eilish's hit when the party's over.

Bridgers had previously covered the track on SiriusXM in 2022, and Eilish had also brought her out as a special guest during a show at Kia Forum last December.

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The unexpected crossover sent music fans buzzing all over social media as they praised the gorgeous harmonies.

"I'm in love with all three of them, truly," the Ocean Eyes singer said after the trio exited.

But boygenius wasn't the only special guest to be welcomed at Eilish's intimate London show, as she also brought Labrinth on stage for a performance of their collaboration, Never Felt So Alone.

Never Felt So Alone first appeared on the soundtrack of HBO's Euphoria before it was formally released earlier this year.

Eilish's invitation comes after she appeared as a surprise guest for Labrinth's Coachella set in April.

The Grammy winner recently dedicated her performance of the song at Lollapalooza to the memory of the late Euphoria star Angus Cloud, who tragically died last month.