Take a moment for a breather, then prepare to explore the bridges leading away from this room/platform.  The right-hand bridge, which you should explore first, leads immediately to a battle.

Battle: Worker Zack (680 Poke)
Opponent: Drilbur Lv 17

After Zack is a dead end, so turn back and head for the other bridge.  Instead of taking it, though, take the side bridge to the West but don’t turn South immediately across the water, because yet another dead end waits there.   The same is true of heading due West, so instead cross more water to the North and go mucking about there and locate a Rare Bone.  That done, it’s time to return again to the room with two Northward bridges and take the one on the left that you bypassed earlier.  The far side of the wall behind Zack is devoid of anything, so continue North and cross another bridge to face a Doctor (thought what he’s doing in the sewers is anybody’s guess).

Battle: Doctor Heath (2040 Poke)
Opponent: Sewaddle Lv 17
Opponent Item: Full Heal, Hyper Potion

Doctor Heath will heal your pokemon (as though Hugh wasn’t already doing it- keep Heath in mind if you return to the sewers, though) both when you defeat him and any time you come back to talk to him.  Heath is also standing near a door to the North, but you should turn your attention first to the bridge beside him, which leads West.  Across there is another door- make note of it, for the empty room it leads to is important later.  Head back past Doctor Heath, going East, and you can pick up a Leftovers for one of your pokemon to hold before checking through the door there.

Through the door you can find a small area full of pipes, with a female scientist lurking around in it.  This scientist is running experiments with the toxins found here and in Poison-type pokemon to see if she can use Poison pokemon secretions to make curatives.  You can come back to her every day to see if she’s succeeded- each time she does, she’ll offer you a healing item of some sort.  The rest of the room is devoid of anything interesting or useful, so it’s time to head back to where you saw Team Plasma malingering.  There are a man and a set of stairs just South of where Plasma lurks, but the guy won’t let you past anyways, so focus on the thieving goons and head North after them, once you make a quick diversion to the West to grab the Twistedspoon item lying on the floor.  After this, it’s finally time to catch up with Team Plasma.