Furnace room
You start in a dark and dusty room with steps on the far side. Check to the left of the steps to get 5000 PTAS and get "Our Plan" from the table at the top of the steps. It's pretty much just a summary of events so far from Saddler's point of view. Open the door on the left to start a cutscene. Ada and Krauser seem to be plotting against you, though Krauser is very suspicious of Ada.
You're now in a hallway with steps leading down. Follow the path to the corner to get find Shotgun Shells, then go back and take the steps down to the lower level, defeating a soldiers or two that may come through the door. Shoot the sparkling object in the skylight to the north to get an emerald. There is a Green Herb at the bottom of the stairs.
Go through the door and fight the soldiers among the the large furnaces. Watch for archers on the left and a soldier with a shield.
At the end of the aisle you can go three ways. To the right is a storeroom with 3000 PTAS and a Flash Grenade. To the left, at the top of some steps, is a small office with possibly a holdout soldier. Get Shotgun Shells from a desk and Rifle Ammo from a locker here. Go back and through the remaining door at the end of the aisle.
Power Plant
Go through a hall and the door at the end into a lager room with large turbines. Most fo the room is fenced off though. Grab the Shotgun Shells from thr floor and take the lift to the upper level.
Now it's time for QTE hell part 1. It's a long cutscene where you dual Krauser with knives. But every so often you have to do one of those button combos to do a saving move as the two of you alternately talk and fight. There are six of them strung together and they are randomized. After these there is a part where Krauser jumps on top of you and you must press first one button, then another quickly to push him off. There is a continue somewhere near the middle, so if you get close to then end you won't have to start over from the beginning.
Meanwhile there are a few revelations in the cutscene part. Krauser is another acquaintance of yours but you thought he was killed several years ago. He's the one who kidnapped Ashley in the first place, but it was a way of gaining Saddler's trust. His goal is to get in deep enough with Los Illuminados to get the "sample" for Umbrella Corp. Finally Ada appears and turns on Krauser. He does a superhuman jump to escape, so it appears he may be getting power from the parasites as well.
You were probably watching for button presses instead of what was happening in the fight, so chances are that you didn't notice that you went over a ledge at some point. So you're going to be really temped to climb the ladder here, but that just takes you back to where the fight starts. Instead, jump down to the floor and go through the door.