Digger walks up and asks if he can throw in his two cents. Why not? Digger points out that the map only shows two dimensions, but the killer might be playing his game in three. Digger concludes that the reason no one saw the killer exit the building is because he's below it. Lt. Bosslady realizes that means he could be in the sewers. Digger says that it's a great way to get around the city, and he knows that because he worked there in his youth. Wow, what a coincidence! Good thing Digger was there, and eavesdropping on the conversation! Frank asks if that's why they call him Digger, and Digger says no. Heh. Lt. Bosslady asks how the killer got into the sewers. Digger says they have to figure out where the station's drainage line hooks up with the sewer system. John thinks it must be the morgue. Frank asks how the killer could have slipped down there undetected. Lt. Bosslady says that he could have snuck in during the evacuation. Frank and Lt. Bosslady head to the morgue. John asks Digger what he thinks, and Digger says that he thinks he can find another way down there. Of course he can.
Digger takes John to an emergency exit shaft that's not even on the map, and conveniently comes out in the police station. John climbs down. Digger hands him a flashlight. John tells Digger to tell Lt. Bosslady where he is. John walks through the tunnels and calls Stella's name. He walks by a giant fan that makes me sure this is part of the old Dark Angel set. John walks into a large room and trips over a rope stretched across the entrance. This causes a valve to open. Stella is at the other end of the room, bound and gagged, inside a tank that is filling with water. Then again, it's the sewer, so I hope that's water. Ew. Stella screams. John sees that there is a grid of ropes throughout the room. He tries to cross the room without tripping over any more of the ropes as he explains that when he gets nervous, he tends to spout random facts. So he spouts random facts. Then he realizes that the cable used on Stella is the same that was used to tie up Pesco, and Pesco had access to those cables. John realizes that Pesco tied himself up so that John would save him and he would get access to the investigation. So Pesco's the killer. Which I already told you, and if you watched the episode, you had surely figured out by this point. And John's supposed to be the smart one?
The lights go out. Pesco appears and tells John that he knew John would figure it all out. Pesco is wearing his night vision goggles (which must be issued to all serial killers), so he can see John, but John can't see him. John asks about the call that came into the station while Pesco was there. Um, maybe you should worry less about solving every piece of the puzzle and worry more about saving Stella? Pesco plays the tape of the call. Good thing he just happened to carry that tape around with him at all times in case this situation arose. John tells Pesco to give up because he's caught. Pesco says that you can't catch what you can't see. John tells Pesco to let Stella go because this is between the two of them. Pesco says that John is the genius detective and he is the brilliant crook. John yells out that it isn't a game and Pesco is killing people. Um, maybe a little less talking and a little more saving Stella? Pesco says that they are mere pawns on a chessboard, and there's a fine line between genius and madness. John says he's nothing like Pesco, but Pesco says they are more alike than John knows. The lights come on, and John sees that Pesco is right beside him. They scuffle, and Pesco pulls his shirt aside to reveal his scar -- the same symbol that John has. John makes the crazy eyes. Pesco says that this is to be continued, because Stella is almost dead, and we see that the water level has risen above her head. Pesco says that John will have to decide whether to chase him or save Stella, and then he runs off. John pauses for a minute, and then grabs a pipe off the wall and smashes Stella's tank so that all the water flows out. He unties her and carries her out. She coughs and sputters, but is clearly alive and well. Yay! John looks down the tunnel, but Pesco is gone. Then he looks real hard for about five more minutes, like, cut the scene already unless Pesco is going to pop up out of the water like Robert De Niro in Cape Fear. Which he's not.