Your rival waits at the bottom of the stairs for you to come down and join the hunt for Team Plasma. That's right, this is a multi battle sort of deal, just like from Diamond, Pearl, and Platinum and the Stat Trainers. All the battles in here will be double battles with him here (which means double chance for a Pokémon you may be looking for), and he will attack the wild Pokémon, which is a double-edged sword if you plan on catching anything here. Your rival always has a Level 20, evolved form of the starter that has the advantage over yours (so, Servine if you picked Oshawott, Dewott if you picked Tepig, and Pignite if you picked Snivy.) They aren't exactly squishy, though, despite the sewers being ridden with Poison type Pokémon, and you or your rival potentially having Servine. The path to get through the sewers is different depending on what season it currently is in Unova. Depending on when you play this, you will go on boards on the sewer waters or stairs on the ground. Either way, making your way north and a little bit to the east gets you to some Team Plasma grunts and a TwistedSpoon.
If you choose to go down away from the grunts, there are a few trainers to take down, and more to explore. You can get an X Special by going down and to the left a little during Spring. Going forward and a left at the branch (then straight to a Doctor) leads to Leftovers, an item that heals 1/16th HP every turn if you have a Pokémon hold it. Going inside the alcove near the doctor is a place where scientists are trying to turn poison into medicine, and they give you an item each day as a sign of their efforts. This ranges from potions to Antidotes and the like. Navigating a little more, you can find a small alcove with a Rare Bone in it. You do not want to sell this immediately, as there is someone later in the game who will buy it for a better price.
The only new Pokémon found here is Zubat, which is pretty good later in the game if you evolve it into Crobat. There is a Bug-type Gym coming up, after all.
The Plasma Grunts attack you and your rival with Scraggy and Sandile, but they are not too much trouble. After defeating them, Burgh tells you that they were the only ones there, and your rival gives you HM 04 Strength, an 80 Power, 100 Accurate Normal move. Colress then enters the scene, but doesn't do anything except say a few words and leave. Burgh then leaves, which leaves you to decide what to do next. If it is Spring time, head left (where Burgh entered) and there is a Heart Scale waiting for you across a board. Further exploration of this place requires Surf, in which we don't quite have that yet.
Heading into Relic Passage (the opening just ahead of the Grunts) leads to an entire cave system, but you have the option of just leaving the sewers entirely and just coming back later.
From the exit of Relic Passage, walking down a little ways (Spring time, you'll have to make a little bit of a trek when the water isn't raised) leads you to a place where, as a man occupying the place says, "...where it all began for Castelia City." Here, you can find a Miracle Seed, an item that raises the power of Grass type moves. Also, in the grass shaped like a Poké Ball, you can find a few new Pokémon.
First is Buneary in Black 2 and Skitty in White 2. Both of these are Normal-type Pokémon, but they are severely outclassed by the Lillipup family. They have good Speed, but not much else going for them. Both of their abilities are pretty useless as well.
Also a pair of exclusives are Petilil and Cottonee in White 2 and Black 2, respectively. Petilil's family has a shallow movepool, but has access to Sleep Powder and Quiver Dance, so it definitely has a niche as a boosting Pokémon. Cottonee is really fast and annoying, as it can get Prankster for an ability, giving things like Taunt, Stun Spore, and Tailwind priority. Its Sp. Attack is not too bad either, considering it can get Hurricaine and the like when it evolves. Both of these evolve via Sun Stone.
The rarest Pokémon to find in here is Eevee. There is a 5% chance to find one, but everyone knows and adores Eevee. It can evolve into many different Pokemon depending on stones, time of day, place of evolution, all of that. There is someone who just gives out an Eevee somewhere else, with its Hidden Ability, in fact, so if you really want an Eevee, don't stress out because you can't find one here, there is one given to you later.