“Niagra, Niagra” begins quietly in a drugstore in Poughkeepsie, where Marcie, the film’s disarming heroine, likes to shoplift. She literally crashes into Seth, a quiet outsider, also on a shoplifting spree. Marcie invites Seth to accompany her to Canada to find a black hairstyling head. They set off in Seth’s beat-up station wagon, destined for a toy store in Toronto. While on the road, Marcie confides to Seth that she has Tourette’s syndrome, necessitating a series of detours to liquor stories and pharmacies along the roads of upstate New York.