S ounding a power chord of defiance against the milksop trends of pop is this good-natured documentary about metal superheroes Iron Maiden . The origin of the band name isn’t explained, incidentally, perhaps for the fun of letting people get freaked out by looking it up for themselves. It’s cheerful and watchable, if a relentlessly on-brand fan promo, corporately policed and controlled, using vintage archive photos and video rather than closeup talking-head footage of the band now. It is uninterested in anything critical, with fervent, humorous testimonies from Maiden superfans from all walks of life, including Javier Bardem, Metallica’s Lars Ulrich and Kiss’s Gene Simmons. The film tells the story of the massive rise, very slight fall, and then further massive rise of Iron Maiden, whose colossal success was achieved without kowtowing to the smirking media gatekeepers of cool.…