Talk to Edmond behind his house. He is trying to find his daughter, hoping she's still alive. Ask what happened to her, he'll tell you she went over the wall to help those affected by the plague.
When you ask to help find her, he'll tell you that Dwellberries repel the virus, and can be used in a special face mask. If you don't have dwellberries already, go to the north side of McGrubor's Woods and enter through a whole in the fence. Some dwellberries spawn inside.
Once you have them, talk to Edmond again. He'll tell you to talk to his wife inside the house. Give her the dwellberries, and she'll give you a gas mask. If you lose the mask, you can find a spare in the wardrobe inside the house. While inside the house, take the picture off the table.
Go back out and talk to Edmond. He'll tell you to dig into the Ardougne sewers. To do so, use four buckets of water (one spawns nearby, you can simply refill it from the house) and use it on the mud patch in his garden.
After the fourth bucket, the soil is soft enough. Stand on top of it and dig with the spade. You fall through the hole, landing in the sewer. Edmond will follow you. Follow the sewer south, and use the rope on the grill pipe.
Talk to Edmon and ask him to help you pull the grill off. He'll ask you to find a man called Jethick.
Put on your mask and enter the pipe. You will exit through a manhole in West Ardougne. Jethick will be nearby or a little east of where you came out. Talk to him and show him the picture. He'll tell you to check out the Rehnison family house, a small timbered building at the far north side of town.
He'll also give you a book and asks you to return it to them. Go to their house, north of the large building. Try to open the door, and Ted Rehnison will yell saying he doesn't want any. Say you're a friend of Jethick's and he'll invite you in.
Talk to Ted again to learn Elena was jumped and kidnapped before she was about to go. Go up the ladder and talk to Milli. She'll tell you they dragged her into a boarded-up building in the south-east corner of West Ardougne.
Go to the building marked in green on the map above. Try to enter the doors and you'll be told you don't have enough clearance. To get the clearance, you'll have to talk to the Head Mourner or Bravek.
The Head Mourner refuses to help you, so you must go to Bravek. Go into the large building north of the manhole and talk to the Clerk. (marked in pink on the above map).
Ask the Clerk who is through the door, and tell him that you need to see him about something urgent. Go up the stairs and into the east room. Try speaking with Bravek, and he'll have a hangover. He'll give you a piece of paper with the ingredients for the hangover cure.
Use chocolate dust on the bucket of milk to get Chocolately milk. Then use the snape grass on it to get the cure. Use it on Bravek, and he'll then speak to you. Tell Bravek the mourners won't listen to you, and he'll give you a warrant.
With the warrant, go back to the plague house. While the mourners argue about you having the warrant, you'll sneak in.
Go down the stairs and try to open the door. The door will be locked, and Elena will tell you they stashed the key somewhere. Go up the stairs and search the barrel on the west side of the stairs. Go back down and open the door.
Talk to Elena and tell her how to escape. She'll tell you to go see her father, Edmond. Go back down the manhole, and talk to Edmond.