Plague City

Gathering the Items

 * Dwellberries spawn in McGrubor's Wood west of Seers' Village. You can get in by squeezing through a loose railing on the north side. Additionally, you can buy dwellberries in the Tree Gnome Stronghold or even grow them yourself using Farming.
 * Rope can easily be bought in the general store in Ardougne.
 * Milk can be obtained by using a bucket on a dairy cow.
 * Chocolate Dust can be obtained by using a Pestle and mortar on a chocolate bar, which you can buy in Yanille or Port Sarim.
 * Snape Grass is most easily obtained on the small peninsula behind the Crafting Guild.

Starting the Quest
Once you talk to Edmond, he'll tell you to talk to his wife about getting some sort of protection against the plague. His wife is just inside the house, and she'll tell you she can make a gas mask if you get her some dwellberries. Give her the berries, and she'll give you a mask. If you need a spare, there will be one in the cupboard. Also, make sure you get a picture of Elena while you're there.

Talk to Edmond again, and he'll tell you about his plan to dig under the wall and get into West Ardougne. First you'll need to soften the ground, though. Use your buckets of water on the odd patch of ground behind his house to get it all muddy, then use the spade you can find nearby to dig through into the sewers. You'll find Edmond inside as well. Walk further inside, and you'll find a pipe blocked by bars. Talk to Edmond, and he'll suggest you tie a rope on it so you and he can pull it off together. Just use a rope on the pipe and talk to him again to pull the grate off. Put on your gas mask and climb inside to go into West Ardougne.

Finding Elena
Once inside West Ardougne, talk to Jethick. He's standing nearby. He'll tell you to go check the Rehnison house and give you a book that he borrowed from them. Head north and a little west to find the house sitting right up against the edge of the wall. They'll let you in when you mention you have the book. Upstairs, talk to the daughter of the household who'll tell you more about Elena's whereabouts.

Head back out of the house and south of the town square to find the house the girl told you about. You'll recognize it because of a big black "X" on the door. Unfortunately, the Mourners won't let you in because it's been hit hard by the plague. Tell them that you're looking for a kidnapped girl, and they'll tell you you need permission from Bravek to enter the house.

Getting Inside
Go back to the town square and go into the large building just north of the manhole. Tell the Clerk you're there for something very important and he'll let you inside to see Bravek. He can hardly talk to you, though, as he's badly hung over. He'll give you a ratty piece of paper with a recipe for Hangover Cure. To make it, put the chocolate dust into the bucket of milk and add Snape Grass. Give the concoction to him, and he'll give you the permission you need to go search for Elena in the Plague House.

Head back to the house and try to enter again. The Mourners will try to stop you, but when given the parchment from Bravek they'll be rather at a loss, giving you a chance to sneak inside. Elena is in the basement. Talk to her, then go to find the key, which is in a barrel upstairs. Unlock the door and release the grateful girl.

Head back to the manhole to get back outside and talk to Edmond to end the quest.

Rewards

 * 1 Quest Point
 * 2425 Mining experience
 * A magic scroll, letting you use the Ardougne Teleport if your Magic is 51 or above. After you read it, you'll memorize the spell.