"Chasing the Light" was the eighth story mission of Infamous: Second Son.
This mission will start after Reggie phones you. You'll find this mission over in Pioneer Square, by a store named Cole McG's Electronics. Prior to beginning the mission, you must take out the D.U.P. Mobile Command Center within that area.
After the command center is destroyed, head back to where the mission starts, and Reggie will phone you. You must first investigate a crime scene, but do not kill any cops.
Head into the alleyways until you approach a dead end littered with neon. Here, you'll be prompted to take a photo by pressing 'up' on the D-pad. This will bring out Delsin's phone. There should be a slider below the picture area. When full, a side panel will appear and allow you to snap the photo using R2. Snap a photo of the graffiti and the man hanging upside down within it.
Next, you'll be informed of a second crime scene. You must clear out the D.U.P. agents before you can investigate. Steer clear of the heavy D.U.P. soldier here as well. Once they're down, locate the fountain structure covered in neon. Here, you must take another picture with Delsin's phone. Once you do that, you must now search for the dealer's stash and destroy it. You'll find it in a duffel bag right next to the crime scene.
After you destroy the stash, a beam of light will shoot out. Chase after it until you realize you can't keep up. Head back to the second crime scene, and climb your way up to the neon sign the sniper was using. You must take photos within this small place to prove that the sniper is female. Move a bit further into the sign to snap a photo of the neon face burned into the wall. After that, you must go investigate the whereabouts of the murder.
Head over by a theater where you'll find some drug dealers. Take them down and then enter the theater by dashing through the gate on the ground floor. There will be a bunch of neon graffiti around the place. Snap a photo of the altar, and the section with a chair and a table with food on it. After that, return to the outside to complete the mission.