First, The Cloud is a sweet (but not simpering) teen romance. Then it's a terrifying ecodisaster thriller. Then it stitches the two genres together in a surprising and heartbreaking way. There is a brooding heartthrob, a tragic heroine, mean boys, a nuclear accident, mass panic, mass death (the scenes of mobbed train stations and gridlocked highways are chilling), an epic bicycle escape, fatal rain, and young, irradiated love. It's a German take on the Hollywood blockbuster and it's ausgezeichnet. (Pacific Place, 600 Pine St, 324-9996. 6:30 pm, $10.)