When Lady Gaga flounces onto a concert stage in one of her Marie Antoinette-gone-Clockwork Orange getups, there's never much doubt that you're watching a glorious ''monster.'' The magic of Katy Perry, by contrast, is that she'll show up to perform in a shiny blue wig with Bettie Page bangs, wearing oversize spinning lollipops pinned to her breasts...and damned if she still doesn't look like the girl next door. In the thrilling 3-D concert film Katy Perry: Part of Me, there are a lot of things to love about Perry, from her sweetly sexy and pert beauty to her full-throated voice to the outrageous hookiness of songs like ''Teenage Dream'' and ''Last Friday Night'' (to me, her greatest track).
Yet you could talk about every one of those things without capturing what's most special about Perry: the joy that beams out of her like a holy light. Part of Me works hard to prove it's more than a glorified infomercial, and one reason it is more is that Perry has a startling story to tell. The daughter of Pentecostal ministers, she grew up in a Christian bubble, sheltered from pop. But once she embraced pop, she did so with an almost religious zeal. The film also documents the breakup of her marriage to Russell Brand, and though it's glimpsed from the sidelines, Perry's despair is on full display. Part of Me demonstrates why feeling the sorrow may help to project the joy. A-