Dr. Jason Cole is a highly respected neurosurgeon who has it all - a lucrative career, confident charm, and the gift of compassion. But he also has a deep, dark secret. One morning, when he wakes up disoriented in a wrecked hotel room amidst several near-naked women he's never seen before, he knows one thing: it's happening again. 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."
Jason's attempt to control Ian's spending backfires when the doctor comes to in a random sports car tailed by a drug dealer. Meanwhile, a falling out between Jason and Lena prompts a late-night visit from Ian; Dr. Jordan tries to get to the bottom of Jason's behavior; and Olivia worries about her son's new imaginary friend.
Ian breaks an important rule by beating up a man in a bar brawl, which puts Jason's medical skills to the test. Meanwhile, Jason tries to mend his relationship with Lena; and Olivia is unnerved by Ian's stalking.
Jason makes a deal with Ian that he can see Olivia, but only on the condition that Ian pose as Jason at a formal affair to honor Dr. Young, though the evening is nearly ruined by Ian's behavior, which pushes Lena away and puts Young's daughter in danger. Later, Olivia meets Ian in a hotel room, but it's a rendezvous he may soon regret.
Ian has a new love interest whom he met in D.I.D. therapy, but Jason worries about her health, and when he tries to help her it might mean the end of Ian's romance. Meanwhile, Dr. Jordan tells Dr. Young that someone altered her daughter's chart; and Lena pursues a new relationship.
Jason receives a text message threatening to reveal his secret - but who sent it? Jason's search for the sender reveals there's a thin line between ally and enemy. Olivia, on the run from Ian, takes Cole to a place overrun with memories of an earlier life with his father. Closer to home, Jordan's investigation into Jason's past turns up a promising lead.
Ruben arranges for a gang to "mug" Ian so that he can get a sample of his spinal fluid -- a necessary element for his pending kill drug, but the plan backfires when Jason is blackmailed by the gang into potentially killing a patient who is a member of an opposing gang.
When a surgery's complications force it to continue after 8:25pm, Jason chances taking Ruben's still-unfinished kill drug to hold Ian off long enough to finish the surgery. The surgery is a success, but the drug doesn't last as long as planned and both Lena and Olivia suffer as a result.
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.
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.
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.
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.
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.