Thor (Chris Hemsworth) and Loki (Tom Hiddleston) unite uneasily for Ragnarok, a world-destroying battle with their evil sister Hela (Cate Blanchett). The movie would be better if it were a half-hour shorter—director Taika Waititi, stepping up from his quirky indies roots, keeps getting swamped by the scale and letting the numbing CGI battles get away from him. But when he asserts his impish side, the movie perks up; at it’s best it’s a knowing self-spoof that doesn’t take itself too seriously. Hemsworth is allowed to re-deploy the surprising gift for comedy he revealed in last year’s Ghostbusters, and the movie profits from it. Another plus is Tessa Thompson as a new character, Valkyrie, a sort of female Han Solo; she’s a much better match for Hemsworth than Natalie Portman in the first two films. J.L.