Your first objective is marked on your mini-map as a light blue dot. Head toward it and find the body of a woman. Press 
to pick up the echoes of past events. You then have to follow a "ghost" across town; don't worry if you lose it, just press 
to bring it back. Watch out for a new type of Reaper, the Conduit. This electrically-charged enemy wears a long white trench coat and can teleport short distances as well as create an electrical shockwave that tears up the ground (and you).
Chase the ghost[edit]
You'll run through a few alleys and then approach a wide street. Back up, because there is a whole mob of Reapers waiting for you. Duck back down the alley, take cover and wait for the Reapers to come to you so you can pick them off one-by-one.
Follow the ghost down a few more alleys and across another street before approaching a large electrical station. Again back up and use caution, because a large group of Reapers awaits you. The station is full of explosive junk, so start blowing stuff up. You'll probably take a few Reapers out in the process, and you'll make the large generators safe for you to take cover behind.
Beyond the station, watch the cutscene and notice how there is now very little left in the way of electrical sources to draw energy from. And you have no power left for anything other than your basic lightning bolt. Fortunately, you don't need to fight anything just yet. Use and follow the ghost out into the street to a manhole cover. Press to enter the
sewers and do a bit of platforming over to a partially lit power transformer. Step up to it and press to re-establish the circuit.
New powers[edit]
You just learned the touch-based powers (Pulse Heal, Arc Restraint and Bio Leech). Follow the cries for help and heal the guy over on the platform. His partner opens the gate for you and you can continue through the sewers. Jump over a few bridges to a large platform with a number of injured Reapers. Use Arc Restraint to secure two and Bio Leech the other two.
Continue on, jumping to bridges and pipes, etc. until you get to a gathering of Reapers. There's a large column you can use for cover right before getting to the Reapers' platform. Also note if you knock a Reaper into the water, you can electrify the water and take out the Reaper. Keep going until you find the missing electrician.
To play the good guy, step up to the gate and press to give Brandon the bad news. Otherwise, just fry the guy. Go through the now open gate and platform your way over to the substation. Zap the panel with the red, flashing light and you'll light up a whole area of the city.