Dr. Frances Sobel, the director of Camp Sumertree, a camp for disabled teens, calls in the OSIR to dispel rumors that recent bizarre incidents were the work of a legendary lake monster. Sobel hopes the team will disprove the myth which could ultimately hurt the camp's reputation.
Most recently, campers believe the monster caused the violent rocking of a boat on the lake. Donner and Axon meet with the young campers, Abby, Tyler and Clay, who witnessed the boating incident. When Donner meets with Tyler, who is autistic, she is intrigued by some drawings he shows her. One is a picture of the lake ??lm except for some unexplained bumps on the water. The other is a puzzling picture of two people ??e large, one small.
Tyler is drawn to Donner, and seems to respond to her. Donner is captivated by the apparent connection with Tyler and curious about what's going on inside his head. While walking together on the campgrounds, Donner and Tyler come upon Abby, who had been chatting with Axon earlier. Suddenly, Abby's wheelchair levitates three feet off the ground and hovers in the air.
When tests reveal that both Abby's wheelchair and the metal boat register electromagnetic frequency (EMF) readings, Axon and Donner shift the focus of the investigation. They don't believe the incident on the lake had anything to do with a monster, but rather was caused by someone displaying psychokinetic ability. This same person could have rocked the boat and lifted the chair.
They conduct tests on Tyler and Abby ??o were both present during the two incidents. It turns out that Tyler displays the psychokinetic ability. He also seems to be experiencing intense physical and emotional pain. Dr. Sobel is shocked to discover the cause of his physical pain is a large lump on his side. Axon places a call to Tyler's begrudging father, Kyle Markson, and rushes a sample from the lump to OSIR headquarters.
Tyler's emotional conflict seems to be caused by another entity ??second voice coming from within him. While Tyler is only able to emit repetitive words of pain, the second voice cries for help. Tyler resists seeing his father, a cold, unsympathetic man while his second voice begs to see "daddy".
As Tyler's two voices argue bitterly, Kyle is stunned to hear his son utter the words "No Tim." Kyle explains that "Tim" is the name they would have given Tyler's expected twin. But the second embryo disappeared and only Tyler was born.
A second shocking discovery reveals that the lump in Tyler's side is not a tumor, but the living embryo of Tim. Tyler's twin is growing entirely within the body cavity of its fraternal host. The psychokinetic force is coming from Tim who is desperately trying to break free from his brother's body. Tormented by the growing will power of Tim, Tyler runs off. Donner and Axon must find him before he harms the brother who is causing him such bitter conflict and physical pain, and, in the process, harm himself.