Twenty year old Abby doesn’t have that much longer to live. She was diagnosed with heart failure nine months ago, and the doctors said she only had seven. She doesn’t want to think about leaving her one true love, Jason, behind, or her father who has dementia. As she withers away in a hospital bed, she writes what she believes to be her last letter to Jason, saying she would have accepted his proposal, married him, and had his beautiful children if not for this. But her savior comes; a young woman who has a heart that is still working and a brain that is not. And it is a perfect match. The nurse lets Abby see her.
After an impossible goodbye to Jason, the doctors put her under. She awakens in water up to her knees in the dark. She is very much dead. The spirit of Rosie greets her, a beautiful girl with long hair and green eyes. She begs for Abby’s help. Before she can say what for, Abby vanishes from the place people go when they die. Doctors were able to restart her new heart after four minutes.
She learns slowly over the next few months that she’s starting to change; her mannerisms and her tastes are targeted first, and then her thoughts. Fearing she’s about ready to have a psychotic breakdown, she decides to leave home for awhile. She hears someone speak in her mind; Rosie. She makes one daring request of Abby; to solve her murder that was made to look like a devastating accident. She ends up at her sisters who she hasn’t spoken to in years. Penelope fears her she has schizophrenia when she tells her what they must do. She offers her sister help after she refuses it medically and threatens to go off alone.
They begin in the parking lot where Rosie was murdered and she recognizes the killer's car. She finds an unsealed envelope to a Miss Corey Jacobs on the dashboard with an address and a note inside that said, “I’m sorry.” They are shocked when they decide to go to Corey's home, and see a picture of her on the wall through the window. That same girl was the one Abby saw lifeless in the hospital. Abby goes to the nearest hospital to see if there’s any record of a Corey Jacobs in the system. The receptionist’s only able to give her the name of the doctor on the case, who took a permanent leave of absence.
She finds him, working at an old café, and arranges a visit, to learn that a man by the name of William approached him two years ago. He demanded for a particular girl’s heart, one with the greenest eyes he's ever seen, to go to Corey, guaranteeing it would be a perfect match. If he didn't, he would hurt his family. Abby realizes she is the rare recipient of a double heart transplant and that Rosie was killed for her heart.
William is waiting at the door to their hotel when she gets back. He's been following her every move every since he saw her at his van. He kidnaps her and brings her to his house, where he presses a gun to her. She finds out that Corey couldn't live with what he had done, and killed herself. He pulls the trigger, and she hears it fire. William shot himself in the head.
After the police brief her, Jason picks her up. They leave flowers on Rosie's grave, as her voice vanishes from her forever with the setting sun.