For a film that’s almost completely incoherent for most of its first act — and seldom finds its footing after that — “Captain Marvel” is rather clear about its one big idea. Jude Law, playing an alien military commander named Yon-Rogg, mansplains it to his promising female protege as they spar during the opening scene. “There’s nothing more dangerous to a warrior than emotion,” he barks at Vers (Brie Larson). “You have to let go of the part of yourself that makes you vulnerable.” He’s Morpheus, she’s Neo, and this is the dojo fight from “The Matrix” if it were less visceral and more explicitly gendered.

But “Captain Marvel,” despite expectations and appearances, is not a groundbreaking science-fiction saga from…