For several decades, the 90-seat basement space in Deep Ellum has staged avant-garde plays by emerging writers. This season, however, something clicked on a higher level with the world premiere of Gordon Dahlquist's sci-fi drama Tomorrow Come Today, the tightly focused work of actor Shannon Kearns Simmons as the title character in The Testament of Mary and the impeccably acted and directed (by Blake Hackler) The Flick, Annie Baker's Pulitzer winner about three nobodies working in an old cinema. Undermain's married founders Katherine Owens and Bruce DuBose have lined up another challenging season, including Eugene O'Neill's Long Day's Journey into Night, which regional theaters rarely touch (it opens early next year), and the current premiere (through October 17) of Meg Moroshnik's The Droll (Or, a Stage Play about the END of Theatre), about a time when all theater is banned and one troupe puts on a secret Hamlet. End of theater? Not at Undermain.