Len Levitt is a subway worker who was shot to death in the park. Light Bulb Len, as he was known at the tracks, used to change bulbs in the stations and tunnels around town. Esposito and Ryan find maps, high-tech gear and surveillance equipment in the victim's apartment. He may have been tracking someone with the initials "BHS."
All that fancy equipment in Len's place belonged to an insurance actuary named Byron H. Singer, aka "BHS." Singer told Len that the rare red-tailed hawks were sighted in the park. That's why bird-watcher Len had the equipment.
Castle realizes that if Len was in the park to verify the sighting of the hawks, he would have been armed with a camera. Actually, Len's camera is found mounted in a tree. The dozens of bird shots end with a photo of man holding a gun on a young boy. Len witnessed a kidnapping. The abductor shot Len when he saw him in the tree. Now the search is on for the unidentified boy.
Castle notices cleat marks on the socks the kid was wearing in the photo. The team does a search of soccer leagues and identifies the kidnap victim as 12-year-old Tyler Donegal. The boy's parents are divorced. Tyler's father, Dean, didn't take the breakup well. He's been a no-show at his custodial job for two days. The team tracks Dean to a motel near the airport where kidnappers told him to wait for further instructions. Dean was also told not to contact the police or his son is dead.
The kidnappers let Tyler to send a video message. At the end of the clip, Tyler says don't forget to feed Ace. Tyler's mom and stepdad confirm that Tyler doesn't have a pet. Dean says his son is a smart kid. He was sending them a message. Before anyone has time to digest this, a text comes in telling Dean to be at the building where he works in 30 minutes. The police realize the kidnappers want this guy because he has elevator access to all the floors in the building. Castle says he's a human key.
Castle realizes that Ace is actually A, C and E—as in the letters denoting subway lines. The last stop for those three lines is at Dean's building. The kidnappers are holding the boy right under their noses. Dean is wired for tracking in a surveillance van out on the street as Castle and Beckett comb the subway tunnels below.
The kidnappers force Dean to take them to a financial firm on the 39th floor. They'll kill him as soon as they get what they need. Captain Montgomery knows that if he moves too soon, he could also lose the boy. Dean does exactly as ordered by the kidnappers. They ride the elevator to the subway level where the bad guys plan to kill Dean. But the car stops at the lobby where Captain Montgomery and his team take down the kidnappers. Dean wants to know what's happened to Tyler. No one has that answer.
All contact with Castle and Beckett has been lost. They are simply too far underground in the subway tunnels. The good news is that they located the boy. Unfortunately, he's being held behind a closed door with a gunman inside. Beckett knows the kidnapper will kill Tyler before she could get a shot off.
Light Bulb Len took his job very seriously. He was instrumental in forcing the city to implement an electricity kill switch in all subway tunnels in case of emergencies. Castle knows the flashlight-carrying kidnapper will light up like a Christmas tree if they use the switch to kill the ambient light. CLICK! Darkness. Beckett storms inside the room. BANG! BANG! Who got shot?!
The mystery of what happened at the subway ends when Beckett and Castle arrive at the station with Tyler. Dean hugs him with sheer joy. As a fellow father, Castle's also feeling pretty good about this one. He rushes home to Alexis, who was rat-sitting for Ashley. She's been panicked because she lost her boyfriend's cherished pet. Ashley's upset, but also very understanding. Castle has to be happy that his little girl is dating such a cool kid. Besides, that off-screen scream from Martha leads us to believe that Ashley's pet rat is missing no more.