Thelonious! is a syncopated comedy about the American Dream told through the nightmare of a failing marriage. At first glance, Carl and Sandy seem to be on track to having it all. Not quite 30, they have high-paying jobs, a healthy infant daughter, and a beautiful co-op in Brooklyn. But then something throws a wrench into this perfect universe — a guy named Thelonious enters their lives. The problem with Thelonious is that nobody can see him except Carl, who is harassed by this presence, especially when Sandy is around. Not wanting to believe that his upwardly mobile life could be the cause of anything wrong psychologically, Carl does battle with Thelonious for a time, keeping him at bay to maintain his comfy existence. But then one day, Carl and Sandy decide to go on a picnic in the Catskills to get away from it all, and when Thelonious shows up here too, all hell breaks loose.