Once More, with Feeling(2001)

The vampire hunters from Sunnydale find themselves in the world of musicals — a very special episode also released as a standalone DVD, featuring the hit "Walk Through The Fire" in which the stars prove their talent for singing and dancing impressively. During a visit to the graveyard, Buffy suddenly feels the urge to sing. A supernatural power plays a trick upon her and her friends, as suddenly they all begin to sing and dance. Soon the whole of Sunnydale is struck by musical fever.