Elisabeth Sparkle (Demi Moore) is movie star famed for her beauty but now working TV doing aerobic workout & motivational slots on breakfast TV. On her 50th birthday she finds out that her show is to be cancelled and in somewhat of a daze gets into a serious car accident on the way home.
Unexpectedly she comes out of it completely physically unscathed but after she breaks down on the examination table one of the doctors secretly slips her promotional material for The Substance which promises to make a better, younger, version of yourself.
In a tailspin, Elisabeth succumbs to temptation, orders and administers The Substance and gives rise, messily on the bathroom floor, to Sue (Margaret Qualley). The Substance did not overpromise and underdeliver: Sue is perfect. Now the two must 'timeshare' life seven days as one, seven days as the other. There are strict procedures to follow.
No good will come of this.
CW: full frontal nudity, body horror ago-go-go, mutation, objectification of women (but subverting that's the whole point)