What is the plot of The Skriker by Caryl Churchill?

The Skriker is a 1994 horror drama written by British playwright Caryl Churchill. It is, essentially, a combination of surrealism, folklore, mythology, magical realism, and modern prose.
The play tells the story of an ancient shape-shifting fairy that comes from the Underworld to stalk two teenage sisters named Josie and Lily. Josie is put in a mental hospital after she tried to kill her baby by putting it in the oven, and her sister Lily is pregnant. Lily asks the fairy to retrieve Josie's sanity, while the fairy's main goal is to get to Lily's unborn child.
In the story, the Skriker is inspired by the ancient mythological creatures from old British folk tales, and, aside from being able to transform into anyone or anything, it can also grant wishes.
The Skriker is a dark and—at times—absurd drama that covers several socially and psychologically relevant themes such as love, manipulation, possessiveness, sisterhood, mental illness, sin, human nature, and cruelty.