When they were teenagers, Prentiss Mathews and her twin sister participated in experiments conducted by biophysicist Emerson Grant. His objective was to tap into the psycho-physiologic connection between them – their 'twinstinct' – a phenomenon that allowed one to sense when the other was in pain or danger. Now adults, the Mathews twins have distanced themselves from Grant and his bizarre research, but Prentiss is haunted by emerging memories of what took place during the experiments. When a suspicious car accident ends Grant's life, Fairfax County homicide lieutenant Drake Hagerman delves into the doctor's past. He discovers the horrifying reality of what Prentiss and her sister went through under the guise of Grant's research. With her mental well-being threatened by a heartbreaking loss and her burgeoning relationship with Grant’s former assistant hanging in the balance, Prentiss is forced to dig deep into her hidden memories to expose the truth. But the more she learns, the more devastating the truth proves to be.