Rainbow Dash has experienced many crashes, and somehow manages to pick herself up and carry on after a couple of weeks. However when she suffers a crash that would be fatal to any other normal pegasus and somehow manages to survive,and a suspicious Twilight begins to question how Rainbow is still living, Dash starts to have repetitive nightmares and discovers a terrifying secret that she's been keeping from herself. What and who is the thing inside Rainbow Dash?
A sister?
A protector?
An impostor?
A possessor?
And more importantly; What does it want?