Read Prima's free inFAMOUS: Second Son walkthrough.
The mission opens with a lengthy cut scene. When you regain control, follow Reggie until you receive the Smoke Shot power. Use this to shoot down the remaining Tracker Drones. After shooting down all of the drones, find the wreckages and collect the Blast Shards. Three should be on the ground with the final one stuck to part of the barricade as you make your way through the checkpoint.
Move forward and meet up with Reggie at the bus. Try using your Smoke Shot to destroy the barricades. After finding out they aren't powerful enough, follow the on-screen instructions to get the Cinder Blast upgrade. Press and hold the R2 button to charge it up, then release it to destroy each piece of concrete. Keep following the bus until you hit another set of roadblocks, again using your Cinder Blast to destroy them. Because Cinder Blast takes up so much of your Smoke, make sure to absorb more from a nearby burning vehicle.
Continue to move along the road until you're attacked by looters. Use your newfound Smoke Shot to take them out. Try not to use the Cinder Blast if at all possible. It uses up way too much of your power to be worthwhile. If you get into trouble, take cover behind a vehicle or Smoke Dash to safety. Keep in mind that your Smoke will deplete, meaning you'll need find other sources to replenish it. Two of the best options for this are burning vehicles or chimneys. Since you won't find any chimneys in the tunnel, look for a burning vehicle.
With the last of the looters down, Delsin gets to choose between subduing his final adversary to benefit his Good Karma, or executing him and contributing to his Evil Karma. This a choice you will have to make repeatedly throughout the game. Again, we opted to go for Good Karma, but you can make whatever choice that best suits your play style.
Sprint through the tunnel and past the cloud of smoke from the collapsing bridge. This portion of the game is much like the first, aiming to teach players how to use advanced movement controls. Approach the Core Relay and absorb its energy. This will allow Delsin to jump much higher when he's standing on the roof of a vehicle. Continue moving in the only direction you can until you reach another Core Relay. This one will allow you to combine your ability to jump with a new Hover technique. Use these moves together to reach the far end of the bridge and save Reggie, ending this mission.