Fringe/Recap/S01 E08

Season 1, Episode 8:

The Equation
A young boy, who has developed a genius-level aptitude for music after surviving an accident, is kidnapped by a serial abductor who has been responsible for the kidnapping of several geniuses in the past using a flashing light combination that hypnotises people. All the previous victims of the abductor have been driven insane before their return, and one of them is locked up in St Claire, the mental asylum where Walter spent 17 years.

Meanwhile, the abductor is forcing the young boy to work on an equation in music form, using hypnotism that makes him believe his (deceased) mother is encouraging him. To find out more about the abductor, Walter agrees to go back into the asylum and speak with Dr. Kim, one of the previous victims. Kim remembers very little, just a "red castle" and a "dungeon". Peter has difficulty getting Walter back out of the asylum, because the administrator feels that Walter is still a danger to himself and others, but he uses his new FBI credentials to secure Walter's release. Using Kim's information, Olivia is able to identify a house where the boy is being held. She and Charlie save the boy, but the abductor escapes -- with the completed formula -- by using the flashing lights to hypnotise Olivia.

The abductor gives the formula to Mitchell Loeb, who uses it in conjunction with a device that allows him to move through solid matter. Once he has the formula, Loeb kills the abductor.