Dr. Jason Cole is a highly respected neurosurgeon who appears to have it all – but also harbors a deep, dark secret. Every night at the same hour, something inside Jason changes, leaving him almost unrecognizable – seductive, devious, borderline sociopathic. This new man is his dangerous alternate personality who goes by the name of ‘Ian Price.' For years he's kept him in check with a powerful experimental sedative. But now his – their – body has developed a resistance to the serum, setting Ian free once again. And to make matters worse, after being suppressed for so long, Ian is hell-bent on taking revenge on his oppressor. With everyone Jason cares about at risk, he must stop Ian once and for all. Will they find some common ground, or will they bring each other down?
Jason's brain surgery is about to begin, a neurotransmitter implant that will "cut out" Ian for good, however due to the blackout drug he has taken for years, Jason is immune to the normal anesthetic. A much stronger drug is given to Jason and in the midst of his twilight sleep he flashes back to his childhood and the origin of Ian Price is finally revealed. Meanwhile, Jordan and Lena are on separate tracks to uncover Jason's secrets all of which could prevent the surgery's completion.
After Jason collapses in front of Dr. Young, an MRI reveals Jason has a brain tumor - Jason and Ruben's plan has worked, the "tumor" surgery will act as a cover for Ruben's implant to destroy Ian once and for all but Lena may soon discover the truth and put a stop to their plans. Meanwhile, Ian has no intention of going down without a fight and will stop at nothing to save himself.
Ruben closes in on the kill drug but isn't sure if he should use it -- and, if so, who of Jason or Ian he should use it on.
Jason, now aware that he has a son, desperately wants a relationship with him, but Olivia feels it's too dangerous and painful. We learn the backstory of how Jason once saved Olivia from Ian and from herself. Meanwhile, both Ruben and Lena start to question Jason's practices.
An operation on Jordan's son is made even riskier by Jason's ingesting of the unfinished kill drug, which makes Ian's appearances more random than usual.
