Tomou Ishii is suddenly dumped by his girlfriend Momoko. He is heartbroken and stays in bed for three days. When he returns to school, his friends notice that his wrist has been stitched with red string. They believe he attempted suicide over Momoko, but Tomou denies it and doesn't know how the string got there. Tomou isn't able to cut the thread, no matter what he uses. He later discovers many similar stitches over his back and shoulders.
Tomou's family and doctors are puzzled by what has happened, except for his grandfather. He states that it resembles a "thousand-stitch cloth" whereby in the past, soldiers would go to war carrying a cloth that had been embroidered with the stitching of a thousand women. This was meant to ward off disaster. He believes that his late wife made the stitches to protect Tomou, and something terrible is going to happen to Tomou soon.
That night, Tomou has a vision of his grandmother and many female ghosts sewing red stitches into his body to protect him. He thinks it is a dream but, the following morning, the stitches have appeared. At school, Momoko accuses him of sewing himself up to get back at her for breaking up with him. He had always said they were connected by the "red string of fate" that joins destined lovers. Momoko thinks that Tomou wants people to believe that since she "cut" the string, it is now winding up and covering him.
Tomou wakes the following day to find his whole body covered in stitches. Soon, thread begins to wind up across his legs, face and fingers, covering his eyes and binding his body tightly. He decides the only way to save himself is to "mend" the string by tying himself to Momoko. He chases after her but is completely caught up in the string as it grows and grows. Momoko screams for her new boyfriend to save her. He jumps in front of her but Tomou, trying to lasso Momoko with the string, catches her boyfriend and both are sucked into the huge, growing whirlpool of string.
Momoko feels that she made the right decision in "cutting the string" by breaking up with Tomou. She thinks she is safe because she has only a few red stitches in her wrist.