The Treatment Plant & Queen Leech

The Treatment Plant starts out dark. In the Power Room, grab the Handgun Ammo and examine the display on the west wall — it's a power grid you need to fill in. Thinking of the grid as 4 columns by 7 rows, supply power to these three points (each one reaches up to two points away):

(2, 2), (3, 5), (1, 6)

• With power restored, take the elevator in the southeast corner down a floor. Hold your ground against three zombies, then push south through the Rec Room (grab Empty Bottles here if you want more Molotov materials) and down toward an exposed outdoor area.

• Note the powerless forklift and a glinting item on the cliff nearby — you'll need a battery for it later. Continue south into a room with a small empty pool; the handle you need for a door back the way you came sits on the far side of it, unreachable for now. Push on through the southern door to reunite with Billy.
• Bring both characters back to the pool room. Only Billy can shove the blocks, so leave Rebecca at the control panel and take him into the pool. Number the blocks: #1 and #2 are the two brown boxes nearest the ladder, #3 is the grey box.

Move #3 all the way east
Rebecca: rotate the machine right
Move #1 all the way south
Hop over the blocks, move #3 all the way north
Move #2 all the way west, then south until it touches #1
Billy hops onto #1, Rebecca rotates the machine left
Move #2 all the way east
Billy hops onto #2, Rebecca rotates the machine left
Move #2 all the way south
Billy exits, Rebecca fills the pool with water

• The three blocks form a bridge once the pool fills — cross it to grab the Handle.

• Use the Handle on the door with the matching hole, back near where you fought the last humanoid leech. Equip both characters with pistols before entering — you'll need to hold the southwest corner while up to six zombies close in from two directions. Grab the Handgun Bullets once they're down.
• Climb the ladder in that room and have Rebecca mix the red chemical into her Mixing Set, then push through the northeastern door, Shotgun ready, to deal with two Hunters patrolling a narrow path.
• Across from them is a Safe Room — herbs, Aid Spray, Napalm Rounds, Ink Ribbons, and a Typewriter. Save here; the next fight is worth preparing for.

• After the Tyrant falls, flip the switch in the southeast corner to lower a ladder, climb up for the Mother Board, then use it to call a gondola and send Rebecca to the Dam Control Room.
• Lower the water level from the control panel, then bring Billy back outside near the dam (the Lurker Frog may make an appearance here — ignore it) and across to the far side.
• As Rebecca, head into a small lab north of the Dam Control Room. Take out a plague crawler (with the Custom Handgun if you have it), then mix the blue chemical you find there with your red chemical to form Sulfuric Acid, and combine that with the Industrial Water on the table to make Battery Fluid.
• Reunite the team in the room with the large specimen capsules, boost Rebecca up for the Battery, and mix it with the Battery Fluid.
• Back at the forklift, use the powered Battery to activate it, lift Billy on the forks to reach the Keycard, then use it on the door across the dam and discard it.

• After she's driven off, collect the Gold Shaft Key and Silver Shaft Key she drops. Split the team, use the Silver Key on the left panel and the Gold Key on the right, then restock at the save point and gear box behind the opened doors before taking the elevator platform back up to face her one last time.

• Turning the final valve ends the fight — and Resident Evil Zero.