It begins with 15-year-old Minnie Goetz exulting, “I just had sex.” In The Diary of a Teenage Girl — a riveting, resonant first feature from Marielle Heller, with a breakout star performance from Brit actress Bel Powley, 23, as Minnie — you exult along with her. Set in bohemian San Francisco in 1976, the film is based on Phoebe Gloeckner’s autobiographical novel published in words and drawings. Heller uses animation to duplicate the effect. We’re in Minnie’s head, with all its whirling confusions. So much for adult grumbling that Diary condones pedophilia because Minnie lost her virginity to an older dude (Alexander Skarsgård, excellent), the lover of her mom (Kristen Wiig). Heller’s film is saying that this is what it feels like for a girl. Deal with it. Powley is sensational, expertly blending hilarity and heartbreak. Her scenes with Wiig, sublime in her hard-won gravity, are unique and unforgettable. Just like the movie.