Gather the items in the "Things You'll Need" section. It will be a big help to get things before you start the quest to avoid going in and out of Ardougne. Some of these things can be bought or found around Runescape.
Go to the northwest corner of Ardougne and find Edmond. Talk to him and he'll tell you that Elena, his daughter, went to investigate a plague in West Ardougne. He'll then tell you to talk to his wife, Alrena.
Get information about the city from her. You'll need a mask to enter the city that she can make by using a dwellberry. Give the dwellberry to her and she'll create and give you a mask when you talk to her again. You will be told to talk to Edmond again about the garden.
Grab the picture of Elena that is on the table in the house before you leave. Be sure to leave this picture in your inventory.
Walk to the garden and you'll see Edmond there. He'll tell you that you'll need to pour water on the soil. Use your buckets of water on the "Mud" spot. After you pour all four buckets, you have the ability to use your spade on the patch. Both of you will fall into a sewer.
Follow the pipe trail to the southwest. Click and use your rope on the opening. Go back to Edmond and he'll tell you that you need to go alone in West Ardougne. Go back to the opening and put on your mask. Click on the opening and "search" the pipe. This will lead you to West Ardougne.
Find Jethick, one of the town's citizens. Tell him that you are trying to find Elena. He'll tell you that she is helping the citizens of the city. He'll also ask to return a book for him, which will appear in your inventory.
Go north and find a house. You'll find out that when you try to open the door, the people inside will not let you in. However, because you have the book, they'll invite you in.
Have a talk with the people inside the house. They will tell you that Elena has been kidnapped and you'll be asked to go talk with Milli upstairs.
Get knowledge from Milli that Elena is in a southeast building of the city.
Walk to the southeast corner of the city. You'll notice a building with two doors being boarded up with planks. Try open the west door and the mourner will tell you that the plague has gotten into the house. Tell the mourner that a girl named Elena is inside the building and you need to get inside. He'll tell you to talk to a guy named Bravek.
Walk north of the building to a huge house. Talk to the clerk inside and say that it's urgent to talk to Bravek. The clerk will allow you to talk to him.
Open the door on the right hand side of the room and talk to Bravek. He has a headache from a hangover, so ask him what you can do about a cure for it. He'll give you a poorly, written note.
Follow the note as you make the cure. Use your pestle & mortar with the chocolate bar and add the dust to the bucket of milk. Add snape grass to the concoction. Talk to Bravek and give him the concoction which will cure his headache. He'll give you a warrant when he learns that the mourner didn't listen to you.
Go back to the southeast house and attempt to open the door again. You'll get caught by the mourner again and he'll tell you to ask the head mourner. Open the door and sneak inside the house immediately and quickly when you see the message, "You wait until the mourners' back is turned".
Go down the stairs and attempt to open the door leading to Elena. She'll tell you that there is a hidden key. You'll find the key in an upstairs barrel. Use the key on the door and Elena will tell you that her father will reward you for rescuing her.