Reaching the Queen Leech's lair | Treatment Plant Walkthrough Resident Evil Zero HD Guide

The door that you need to open using the Keycard.

Go back the same way, to the storage where Rebecca obtained the battery (the one from the shelf near the ceiling) (M6,22). Open the southern door (M6,24), collect the supply from the room and head to the western door that are still locked (M6,25). To the left, you can see a card reader. Open your inventory and select (Use) on the Keycard that you obtained after starting the forklift.

At the end of the corridor, you can see ammunition for Magnum.

Go along the corridor and take the Magnum ammunition. Before you open the next door, select some good weapons for your heroes (for example Grenade Launcher, Magnum or Shotgun). After you step into the room (M6,26), the battle with Queen Leech will start.