Hugh gives you away as you approach the Team Plasma goons, getting into an argument with them about whether or not Team Plasma are villains- the goons use this opportunity to attack, prompting a two-on-two battle alongside Hugh.

Battle: Team Plasma Grunt x2 (2560 Poke)
Opponent: Sandile Lv 16
Opponent: Scraggy Lv 16

You and Hugh should make pretty short work of the opposing pokemon at this point, causing Team Plasma to flee ‘like Patrat’, in Hugh’s words.  Just as Hugh prompts you to head further in, looking for more of Team Plasma, a mysterious voice declares that unnecessary!  As it turns out, this is Burgh, who has finally resurfaced (in the nonliteral sense) after running out to chase down Team Plasma in the first place.  Greetings are made, and Burgh explains that no more of Team Plasma remains here.  Complaining that the sewer is ‘bugging’ him, he requests that the three of you head back topside.

Hugh turns and thanks you, giving you the HM for Strength into the deal as a gift.  Re-vowing his eternal enmity towards Team Plasma, Hugh heads out, leaving you alone with Burgh.  This leaves you Hugh-less when a strange man with incredibly strange hair appears out of the hole in the wall North of you.  He compliments you on your battling, then takes his own leave, puzzling Burgh just as surely as he has you.  Pointing out that you can continue to train down here or seek him out for your gym battle, Burgh takes his leave at this point.

There’s more sewer to explore, so you may as well continue on down here.  The hole in the wall leads to a cave called Relic Passage, where you can find Roggenrolla, Onix, Woobat, Drilbur, Rattata,  at levels 16-18.  Roggenrolla is essentially a replacement Geodude, down to its evolution condition, though it lacks the ground typing.  This is a benefit in some ways, cutting down its number of dual-weaknesses a lot, but a hindrance in others, leaving it open to electric attacks as a particular threat given its poor Special Defense.  Onix isn’t really anything to write home about either, with massive defenses and good speed but crummy attack, trading its good speed down to poor and its crummy attack up to adequate on evolution.  More noteworthy are Woobat and Drilbur, as Woobat is a fast and specially-offensive flying/psychic pokemon with some interesting and unusual moves (such as Heart Stamp) that give it its own niche.  Drilbur is most noteworthy for evolving into the ground/steel monster Excadrill, whose high speed, toughness, and titanic attack combine with either of its potential abilities to make it a terrifying sweeper in virtually any situation.

Take the Easternmost path north and you can head down some stairs to a lowered floor area.  There, you will find a scientist eager to battle!

Battle: Scientist Lumina (816 Poke)
Opponent: Venipede Lv 17, Koffing Lv 17