“Dark” is a descriptor oft applied to composer and lyricist Stephen Sondheim. And Passion (which includes the line “Unhappiness can be seductive”) is quirky, even by his standards. The setting is 19th-century Italy, where handsome soldier Giorgio (Matt Surges) is posted to a drab mountain garrison—separating him from hot love interest Clara (Courtney Glass) in Milan. Soon, Giorgio meets Fosca (Jackie Vanderbeck)—as a perpetually ailing, twitchy recluse who obsessively presses for Giorgio's affection. It's a complicated, warped, manipulative relationship. Sondheim's score is elegantly beautiful—much closer to “art music” than uptempo Broadway, and the singers are backed by a spare but effective combination of piano (Graham Sobelman) and percussion (Brian Manchen). Artistic director Connor Mickiewicz stages this strange love story sensitively, quietly and convincingly. Th, F 8pm; Sa 2pm & 8pm. Through 3/8. $30. New Helvetia Theatre, 1028 R St.; (916) 469-9850; www.newhelvetia.org.