Author note: Written by Jachin, a 23-year Resident Evil series fan based in Seoul, South Korea. This guide is based on 6 full RE9 playthroughs on PS5 and Steam (Casual → Insanity → NG+ Insanity completions). All strategies, timings, and locations have been personally tested across the v1.02 patch. AI tools were used only for grammar review and note organization; no AI-generated content appears in strategy analysis.
Verified on: v1.02 + DLC patch · Last reviewed: May 30, 2026 · About the author · Editorial policies
The Water Treatment Plant is Grace's first major sandbox boss area. This RE9 walkthrough covers the three-sunlight-unit restart sequence, valve puzzle solutions, The Girl boss encounter, and the transition to Raccoon City.
| Location | Water Treatment Plant (Grace section) |
| Playable as | Grace |
| Items obtained | Valve Wheel, Maintenance Key, Filter Plate, Sunlight Lens |
| Bosses | The Girl (sunlight-weakness boss) |
| Trophies | Light of Day (Silver) |
| Difficulty | ★★★ Hard (7/10) |
The Water Treatment Plant is the campaign's first true puzzle-heavy boss chapter, and the first chapter Grace plays solo since the Wrenwood Hotel intro. The objective is simple in concept — restart three sunlight units to expose The Girl's weakness, then beat her — but the execution requires you to plan three valve sequences in the right order while the lights are out.
Bring at least 30 SMG rounds, 2 First Aid Sprays, and the upgraded Mortal Edge Hatchet if Leon had it equipped last chapter. The hatchet carries between characters.
Grace climbs out of the sewer hatch into the plant's main filtration room. Power is offline — the only light is from your handheld lantern. Two wanderer zombies in this room; both are blind and can be stealth-killed with a single hatchet swing if you stay behind them.
The Valve Control room has three pressure valves (A / B / C) mounted on the wall. Each valve has a 0-9 dial. The wall placard tells you the goal pressure for each tank, but the valves themselves interact — turning one shifts the others. The solution is fixed per difficulty (Normal: A=3, B=7, C=5; Insanity: A=4, B=2, C=8).
Set each valve in this order: A → C → B. The order matters because of the cross-feed plumbing. Get it right and the basement door unlocks.
Through the basement door is the cathedral-like Sunlight Hall — three giant skylight units, all dark. You need to restart each unit to draw sunlight into the central plaza where The Girl waits.
Pull the breaker, then run the manual crank for 5 full rotations. Easy — no enemies.
The breaker is behind a locked grate. The grate key is in the foreman's office on the mezzanine. Three wanderers patrol the path — stealth or hatchet-parry through them.
The hardest of the three. The breaker room is flooded and you need the Filter Plate (looted from a side closet) to drain it. Once drained, pull the breaker and crank as Units A + B.
All three sunlight units active triggers the boss cutscene. The Girl drops from the ceiling into the central plaza. She is fast, hits hard, and is invulnerable except when fully exposed to direct sunlight.
Full strategy in our The Girl boss guide. Quick summary:
The Girl drops the Sunlight Lens on death — required for the next chapter. The plant's main door unlocks automatically.
The east door of the save room opens onto the rooftop access ladder up to Raccoon City's waterfront district. Grace hands off the Sunlight Lens to Leon in a cutscene as he arrives — the next chapter is back to Leon for the RPD + Orphanage sequence.
The next chapter is one of the campaign's most iconic — Leon arrives at the abandoned Raccoon City Police Department, encounters Mr. X for the first time, and proceeds to the Orphanage where the campaign's emotional core unfolds. Save at the plant and brace.
Continue with our RPD + Orphanage walkthrough, or read the Mr. X / Super Tyrant boss guide first.
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