The tale, co-written by Judd Apatow and the director Jake Kasdan, follows the oh-so-familiar trail from Dewey Cox’s life-altering childhood trauma to sudden fame as a country singer and accompanying substance abuse, through late-career woes and, finally, sober reflection and redemption. There’s little subtlety here — twice Kasdan gets laughs simply by shooting an exposed penis, and there’s a chimp — but even the worst groaners go by quick, a la Airplane, and there’s dozens of star cameos to chuckle over. John C. Reilly throws his all into the naïve buffoon Cox, portraying him from adolescence through retirement. The outrageous hairdos and outfits help (as does Reilly’s occasional resemblance to the late country crooner Conway Twitty). Dewey is mostly an extended skit that will resonate best with fans of popular music, who’ll dig the winks to Cox’s Dylan phase; his drug-addled, never-completed orchestral masterpiece; and his execrable 1970s TV variety show.