After a sensitive widow (Kirsten Dunst) and her enigmatic, fiercely loving son (Kodi Smit-McPhee) move in with her gentle new husband (Jesse Plemons), a tense battle of wills plays out between them and his brutish brother (Benedict Cumberbatch), whose frightening volatility conceals a secret torment, and whose capacity for tenderness, once reawakened, may offer him redemption or spell his destruction.