After her husband Martin completes a four-year prison sentence for insider trading, Emily Taylor drives into a wall in an apparent suicide attempt. Jonathan Banks, her assigned psychiatrist, prescribes a series of antidepressants, but none work. Jonathan contacts Emily’s previous psychiatrist, Victoria Siebert, who suggests an experimental new drug, Ablixa. The drug seems to help Emily but gives her sleepwalking episodes as a side effect.
One night, Emily stabs Martin to death while sleepwalking. Jonathan fights for Emily’s acquittal in court. She pleads insanity and is declared not guilty on the condition that she stays in a psychiatric hospital until cleared by Jonathan. The publicity destroys Jonathan’s reputation, and his colleagues assume negligence on his part.
Jonathan discovers evidence that Emily is lying; she was not depressed and faked her suicide attempts. He also discovers that someone may have profited from Ablixa’s fall in stock value. He interviews Emily after administering what he claims is a truth serum that will make her drowsy. Though the serum is actually saline water, she feigns drowsiness, confirming Jonathan’s suspicion that she is deceiving him. When he confronts Victoria with his findings, she mails photographs to his wife Deirdre implying that he had an affair with Emily. Deirdre leaves him, taking her son with her.
Jonathan calls Victoria’s bluff by telling her that Emily told him about their plot. He threatens Emily with electric shock treatment and tells her that Victoria is paying him to keep her incarcerated so that she can keep a bigger cut. Emily explains that she hated Martin for losing their opulent lifestyle and began plotting to kill him. She began seeing Victoria for counseling, and the women became lovers. Emily taught Victoria about the financial world, while Victoria taught Emily how to fake psychiatric disorders. They plotted to kill Martin and to use the negative Ablixa publicity to manipulate stock prices. Jonathan accepts Emily’s offer to give him a cut of her money if he releases her from the ward.
Emily meets Victoria while wearing a wire. After Victoria mentions details of the plot, she is arrested for conspiracy to commit murder and securities fraud. Emily, due to double jeopardy, cannot be charged as criminally responsible for her part in Martin’s murder.
As retaliation for Emily’s part in the plot, Jonathan, who still oversees her case, prescribes her Thorazine and Depakote and describes their unpleasant side effects. She is sent back to the mental ward for refusing treatment, and Jonathan regains his family and reputation.