“The Riddle of the Sphinx,” a new drama by Amy Losi. Directed by Amanda Cannon. Starring Mary Sheridan and Jose Lopez. A young art critic strikes up a flirtation with an older woman who runs an art gallery. A riddle leads to the revelation of a decades-old secret, unraveling both of their lives. Gene Frankel Theatre Festival, Block 2: 7/31, 8/6, 8/10. Additional performance: Block 9: 8/2.