The Underground Laboratory

• Exiting the Observatory, grenade launcher ready, push through a bat-infested path to the front of a locked church. Split the team: send Rebecca into the small side room to step on a floor switch, then have Billy open the church doors from outside while she holds it.
• Inside, grab Napalm Rounds, Shotgun Shells, and an Ink Ribbon in the save room before pressing on.

• Use the Hook Shot to climb up through the hole it left behind. At the back of the roof, climb down a ladder and flip the switch that powers the western elevator, then unlock the fence gate to the west (grab the Red Herb in the backyard if you want it). Regroup and take the elevator down.

• In the cramped underground Library, save if you need to, then have Billy use the Hook Shot at the hole in the ceiling to reach a closet above.
• A humanoid leech waits inside, still untransformed — walk until the camera centers behind it, throw two Molotov Cocktails at its feet, then finish with a third if it's still moving. Out of Molotovs? There's a Gas Can and Empty Bottles right there to mix more.
• Grab the notes, the Laboratory Map, and the Leech Capsule where the leech stood. Drop the Hook Shot if you need the space — you're done with it for a long stretch.

From the B1 Hall, four differently colored doors marked with the Bust of Marcus wait around the level, each needing its own key item.

• The northern room has minor loot; the western room has a zombie guarding notes with a future puzzle's answer and more Molotov materials, plus a small Gas Chamber.
• Push west through a room with two or three zombies to reach the green Bust of Marcus door and a service lift connecting back to Rebecca. Send her the Leech Capsule and some Molotovs.
• As Rebecca, fight past a humanoid leech in the B2 Hall to reach a Red Canister — mix its chemical with the Green Chemical from the Infirmary (back in the Training Facility, if you haven't already) to make the Stripping Agent. Combine it with the Leech Capsule to get the Blue Leech Charm, which unlocks the blue Bust of Marcus door into the Study.
• In the Study, grab the Input Regulator Coil and Handgun Ammo, then clear two zombies in the next room to find the Green Leech Charm held by a statue. Send it to Billy via the service lift.
• As Billy, use the Green Leech Charm on the green door to reach the morgue. Clear the zombies, grab the Sterilizing Agent and Handgun Bullets, then use the Sterilizing Agent at the gas chamber's control deck to open it and claim the Breeding Room Key.

• Use the Breeding Room Key on the reinforced door east of the B1 Hall. Two Hunters guard the room — a few Shotgun blasts each should clear them. Grab the Dial from the back corner and send it to Rebecca via the service lift.
• As Rebecca, use the Dial to complete a combination lock in the B2 Hall:

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• Through the unlocked door is the stalled Cable Car, just below Billy's platform. Pull the lever on the southern wall to drop a ladder and reunite the team, grabbing the Output Regulator Coil by the car and the Hook Shot on your way.
• Use the Hook Shot in the northeast corner to reach the Cable Car's control room and install both the Input and Output Regulator Coils to power it up.

• Dissolve the leech with Molotovs, return to the control room to reset the fallen Output Regulator Coil, then board the Cable Car. Loot a dead body inside for the Magnum (Desert Eagle), hit the front switch, and ride out toward the Factory.