Christine and Carolyn, they're identical twins and best friends, close enough to be one person. Nothing has ever come between them. But this spring, things have started to change. Carolyn has a boyfriend whom Christine hardly knows. And Christine spends all her time working at the hospital as a volunteer. Lately, she's become fascinated with the hospital pharmacy, until she realizes there's something very wrong. At first it isn't much: an employee who seems especially secretive, a patient who doesn't respond to his medicine. But when a woman dies suddenly for no reason, Christine knows it wasn't an accident. Christine's got to find out what's going wrong, but she can't do it alone. She needs Carolyn more than ever, just when they're farthest apart. Because she's taken a gamble with their lives - and if she loses, they'll both have to pay.