Day 13 follows the story of Colton, a 17-year-old boy who is left at home to take care of his younger sister while their parents are away for the summer. Across the street, Colton notices a beautiful girl named Heather, who has just moved into a mysterious old house with her foster father.
As Colton begins to develop feelings for Heather, he also witnesses her foster father's behavior becoming increasingly threatening towards her. Soon, Colton begins to suspect that the man is involved in a Satanic cult and is planning to ritually murder Heather.
Desperate to protect her, Colton takes matters into his own hands and intervenes. However, he soon discovers that the situation is far more horrifying than he ever could have imagined. As Colton uncovers the truth about Heather's situation, he realizes that he must fight for her survival before it's too late.
