Castelia Sewers
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.
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.