For nine months, Alfie has been caring for his twin daughters after the sudden death of their mother. One night, he wakes to find them at the foot of his bed, and hears the chilling words: Daddy, there's a man in our room. Alfie finds no sign of any man, but it isn't long before the name Black Mamba invades the family's everyday routine. What seemingly begins as an imaginary friend quickly develops into something darker, potentially violent. Alfie turns to Julia - his wife's twin and a psychiatrist - for help. But as Black Mamba's grip on the girls tightens, Alfie and Julia must contend with their own unspoken sense of loss, their unacknowledged attraction to one another, and the true character of the presence poisoning the twins' minds...