LetHerOutfollowsHelen,abikecourierwhosuffersatraumaticaccident.Assherecovers,shebeginstoexperiencestrangeepisodic-blackouts,hallucinations,andnightterrors-thatleadhertodiscoverthatshehasatumor,abenigngrowththatistheremnantsofa"vanishingtwin"absorbedinutero.Overtime,thetumormanifestsitselfasthedarkanddementedversionofastranger.AsHelen'semotionalandpsychologicalstatebeginstodeterioratefurtherandfurther,shebeginstoactoutinpsychoticepisodesthatareinfluencedbyhereviltwin-makingheradangertoherselfandherbestfriend,Molly.