Let Her Out follows Helen, a bike courier who suffers a traumatic accident. As she recovers, she begins to experience strange episodic-black outs, hallucinations, and night terrors-that lead her to discover that she has a tumor, a benign growth that is the remnants of a "vanishing twin" absorbed in utero. Over time, the tumor manifests itself as the dark and demented version of a stranger. As Helen's emotional and psychological state begins to deteriorate further and further, she begins to act out in psychotic episodes that are influenced by her evil twin - making her a danger to herself and her best friend, Molly.

Five university friends gather at a house party to ring in the New Year. Unbeknownst to them, an epidemic has erupted outside, causing outbreaks around the world. With nowhere else to turn, they barricade themselves indoors with only their phones, laptops, and other tech devices. They use their devices to research the possible cause of this outbreak. Information and video footage over flow their computers as they descend further into the cause and the ensuing chaos. As the virus spreads, the mood in the house changes from fear to paranoia. Who is safe? Who can they trust? Reality becomes blurred as they slowly discover the source of the virus causing the sickness...and there is no going back.