An allergic reaction is spreading across her body, seemingly aggravated by Ruth's acts of self-harm. Initially, Sarah mistakes Ruth's condition as a symptom of drug use and is unsympathetic. She treats her wounds and allows Ruth to stay the night on the understanding that she stops scratching.

In doing so, Sarah sets into motion a series of terrifying events that will endanger her life and destroy her treasured home.

When Sarah finally turns to the emergency services for help, she learns that an epidemic has overtaken New York City. The virus, she is told, multiplies through the act of scratching. When the disease reaches the brain, the victims become dangerously hostile and will transfer the illness onto a new host through biting, scratching or violent assault.

The virus needs the infected, as Ruth laments, to be the agents of their own downfall. Sarah must renew her efforts to restrain Ruth, both for her own protection and to stop the sickness from spreading. Her home becomes a fortress, to repel both the advancing forces of chaos, and also the National Guard, which has embarked on a ruthless campaign of containment.

Lifelong quarrels between the friends are settled as Sarah battles a crippling anxiety disorder and searches for news of her missing boyfriend. Dean is lost amongst the bedlam outside, and the question persists, when he returns, will it be as the women's rescuer, or as a deadly new threat?

SCRATCH is a micro-budget horror story that takes place largely inside one house and with two characters. It borrows from possession and zombie films, focusing on a new gestation period in which the terrifying affliction can be seemingly averted. The battle between the two women is therefore psychological as well as physical and themes of friendship, self-destructive behavior, and the character's relationships with their physical selves are explored.