Based on the true story of Samuel Byck, who tried to assassinate President Nixon in 1974, this first feature by Niels Mueller is powerful, haunting, but ultimately disappointing. Few American movies address abject failure as forcefully as this one, and Sean Penn delivers an intense performance as the would-be assassin. Yet casting a glamorous star as a terminal misfit undermines the character’s reality (as in Taxi Driver), and despite fine performances by Naomi Watts as his ex-wife and Don Cheadle as his best friend, one can’t help but wonder why these people would become involved with someone like him. Mueller and Kevin Kennedy wrote the script; with Jack Thompson. R, 95 min. (JR)