Hannah, Hadley Robinson, sits at a table with her parents when suddenly she feels a sharp pain in her side, and a bulge begins to appear. However, it soon stops. Later, she is sewing when she pricks her finger as her boyfriend goes to bed. The next morning, Hannah notices a stressful message on her phone, and the pain in her side momentarily returns. She stubbornly pushes it aside and heads off to work.
At the office, Hannah and her best friend realize she bled on the cuff of a newly created dress, and her boss reprimands her. Running into the bathroom, she sees the bulge beginning to bleed again, but she writes it off as menstrual cramps. That night, her boyfriend tries to get close, but she stops him when the sensation near her side intensifies. She makes up an excuse to leave.
Back home, while drawing, the bulge grows into a small, monstrous head with tiny arms. The head ridicules her until it bursts from her side, and she blacks out. When she wakes, the monster pounces, and she knocks it into a wall. After bandaging her side, she finds inspiration from her blood-soaked clothes and designs a new dress. She brings the blood-red, jeweled white dress to her office, and her boss loves it. A dermatologist visits and explains the skin irritation may be connected to a birthmark and could have been caused by dual DNA from absorbing a twin in the womb.
Later, at a bar with a coworker, Hannah becomes paranoid when her boyfriend lights up her friend’s phone. She returns home to find the monster, who offers to help her. She binds the creature and carries it to the basement.
The next morning, her boyfriend and her coworker arrive together to treat Hannah to a day out, but she feels like the third wheel and suspects infidelity. She searches online and discovers a meetup for people with the same dual DNA condition. At the gathering, several others admit to also having an “appendage” they sedate once a week. Hannah buys the meds and chats with one of the other attendees over coffee before returning home to sedate her own appendage. Kaelin arrives and confronts her about her recent behavior, but she dismisses him. At the office, she yells at Esther. Over the ensuing weeks, Hannah continues to sedate the appendage and spend time with Claudia from the meetup.
One night, Hannah tells Claudia that she once drove her car into a median in high school, wrecking one of her parents’ parties. Claudia suggests you don’t need to sedate the appendage right away, and she listens as the appendage speaks truths about the future. When Hannah returns home, the monster—now claiming Hannah’s form but covered in bloody pus—reappears and reveals a wormy thing that enters Hannah, causing her to blackout and lose her own voice.
Upon waking, the monster has taken over completely. Claudia admits that she is actually the original Claudia's appendage who has seized control of her life. Appendage-Hannah sends messages to make amends with Esther and Kaelin. While dragging the real Hannah into the basement to tie her up, they encounter Esther, who grows suspicious.
As the meetup members reveal themselves to be appendages, the leader explains he has found a way to kill their host without killing them. In the meantime, Esther rescues the real Hannah and takes her to the hospital. Realizing Kaelin is in danger, Esther and Hannah return to the apartment to confront appendage Claudia and appendage Hannah.
After a brutal confrontation, the real Hannah prevails, reconciles with her parents, and stores the now-shrunken, baby-sized appendage in a crib in the attic.
