Learning of the missing cat
Speak to Gertrude in her house just west of Varrock and south of Juliet's house from the Romeo & Juliet Quest. Gertrude's beloved cat Fluffs has gone missing. Tell her you will help if she gives you more details. She says to talk to her two sons about the cat and that they can be found in the Varrock marketplace. Head to the Varrock marketplace.
Shilop and Wilough, Getrude's sons, are located by the fur and Newspaper Stands. Speak to either of them and they will tell you the cat followed them to their secret hideout. They will not tell you where the hideout is so ask them what will make them tell you. You will learn that they are short on cash and will tell you the location of the hideout for 100gp. Agree to pay them and they will reveal the location of their hideout; the Lumber Mill to the northeast. To get in you will have to climb through the broken fence.
The lumber yard is located northeast of Varrock head there and then climb under the broken fence on the west side. This area is marked on the map with a red dash.
Head to the center of the mill and climb up the ladder. You will find Fluffs up there. When you try to pick her up she will scratch you inflicting zero damage. You will learn that the cat must be thirsty. Use a bucket of milk on her. When you try and pick her up for a second time you will learn that she must be hungry. If you try and feed her a sardine she will not eat it as she likes it seasoned. Head back to Gertrude's house and search her backyard for some doogle leaves. Posted Image Use the doogle leaves on the raw sardine and you will get a Doogle Sardine.
Rescuing the cat and a surprise
Head back to Fluffs and feed her the sardine. When you try to pick her up she will not go and will act afraid.
Climb down the ladder and search the crates for the kittens. When you right click the crates the crate with the kittens in it will show up yellow rather than the cyan color you normally see. Once you find the three kittens take them up to Fluffs and use them on her. All of them will run off home to Gertrude.
Return to Gertrude's house for your reward.