Back at Fall City's Base
Joel promotes you to Ranger Rank 3 for doing an excellent job with your mission! With this, you can use the Poison Poké Assist, the Psychic Poké Assist, and lastly, you'll unlock a second bar to your partner's energy gauge, meaning you can now use a more powerful assist with it.
Just then, a civil engineer runs in pushing you to the side, as he reports that the sewers, which are supposed to be clean, are full of sludge! It's infested with Grimer. Joel assigns you your next mission, to go and capture the Grimer and then deliver them to an expert Ranger that can deal with them.
Your next mission has started, so head out of the Ranger Base and go the same way you went to the lab (just north of the Base). Go past the lab, and south to find the sewer entrance. The sewer has a good few new Pokémon to capture, starting with Totodile.
Before you can go on and capture anything else, you'll find some puddles of sludge, and you'll find your first Grimer.
After capturing the Grimer, the sludge will be cleared, and the Ranger that was at the entrance will come running to take the Grimer with him.
Heading east and south to the next screen, you'll find a few more Pokémon, including Corphish which we hadn't seen before. Notice the two metal fence gates.
Heading south and east, you'll find another pair of Grimer. Capture them and continue heading east, you'll find more new Pokémon.
The first two are not necessary, however you must capture a Raticate. After you do, bring it back to where you first saw Corphish, and use Raticate's Field Move to cut down one of the metal fences. Head north to the next screen.
Here you'll find another new Pokémon, Croconaw.
Keep heading east to find another pair of Grimer, capture them and continue moving along to the next screen. You'll find two pipes, you can go into the on the left but it'll only allow you to catch a Drowzee and a Corphish, so head into the pipe on the right. You'll meet with Aria, who will go on ahead with her investigations, go north out of the pipe, and down the stairs directly to the right. Dodge all the Koffing and make sure not to fall down, and climb the stairs on the right side of the room.
Head down the pipe on the left, save and head west and north to the next screen. Crossing the bridge and walking a bit west, you will encounter a Grimer trio. Capture them and head east to see Aria and a small post on the other side of a waterway. You'll need to find a way to cross, so go north to find Tangela, a new Pokémon.
Bring this Tangela down to the place you need to cross and use its Field Move on the post to get across. Before going any further west, go north and capture a Meditite.
After capturing the Meditite, jump down on the side you were previously and go east to find a breakable wall. Use Meditite's Field Move to clear it, and go inside to find not one, but two new Pokémon!
Head back, capture Tangela again and cross the waterway again. This time head west and in the pipe. When you get out of that pipe, go into the one directly to the left of it. In here, go down until you can turn to the left, when you can do go left then down again. You'll see more puddles of sludge if you've gone in the right location, so capture the three Grimer that come down there.
Heading up, you'll see a civil engineer getting attacked by two Grimer and a Muk. Go to him and capture all three of them.
To give you an easier time on this fight, you can capture the Drowzee that appears a bit before this battle and use its Assist, as the Psychic Assist levitates the Pokémon in the air for a little while allowing for a much easier capture.
After the capture, Aria will meet up with you, advising that you should go report that you've cleared out all the sludge from the waterways.