Writing remains the House Theatre of Chicago’s weak suit. Time-travel stories are bound to get confusing, but the narrative of this new play by Chris Mathews and Jake Minton is slight and muddled from the get-go. Despite moments of genuine charm and wit, by the second act it’s disintegrated so completely you’re wondering what its dramatic “conflict” even was, and whenever it delves into the heavy stuff–suicide, grief–it’s just unconvincing. But stagecraft and exuberance have always been the House’s selling points, and this show’s flat-out spectacular special effects represent one more leap forward. Moreover the actors playing the past and future Daves, Steven Taylor and Dennis Watkins, pull off something I’ve never seen onstage: the equivalent of an evil-twin split-screen video trick. Sporting perfectly matched diction, looks, and mannerisms, they’re nothing short of magnificent, and the effect is never less than magical. Another triumph of form over content. Through 7/9: Thu-Sat 8 PM, Sun 7 PM. Viaduct Theater, 3111 N. Western, 773-251-2195. $10-$19.