Fading aerobics star Elizabeth (actress Demi Moore) receives a mysterious offer to try a revolutionary new product - Substance. A cell-replicating product that creates a younger, more beautiful and more perfect copy of a human being. Once you've taken it, all you have to do is follow one rule: share time. Seven days for you, seven for the new version of you. Thus is born the attractive, youthful Sue (played by Margaret Qualley). But the existence of two women sharing one consciousness soon begins to have unwanted consequences.