Lady Gaga joined U2 on stage during their July 26 concert at New York's Madison Square Garden—the band's fifth of its eight-night residency there. And at first, the sight was surreal. The Edge bowed down to Gaga as she sat at the piano, wearing nothing but a black bikini and glittery hooves of shoes. Then, when they launched into a duet version of "Ordinary Love," off 2013's Mandela: Walk to Freedom OST, Gaga raised her left leg as if she was about to kick something down—like she was goofing off. When it was her turn to sing, though, she got serious, sounding like a total grand dame as she unleashed her mighty vocals. To borrow some words from U2, "All hail Her Gaganess." Watch below.