Sylvie and Cassia are twins and grieving he death of their mother Pippa. One night they wake their father Alfie having seen a man in their room. Weeks later they form an imaginary friendship with a black mamba snake. This doesn't seem to worry Pippa's twin Julia but when she hears about the man in their room it is a different matter and as though something similar happened to her and Pippa growing up in the same house, bedroom and sleeping in the same bed.