Head west from Varrock along the road towards Gunnarsgrunn (Barbarian Village) and you will come across Gertrude's house. Head behind it and pick up a Doogle leaves if you haven't done so already. Go inside and talk to her; she will tell you that she has lost her cat named Fluffs. She asks you to go and talk to her children, Shilop and Wilough, in Varrock Square.
Travel to Varrock square and speak with the children. They will tell you that they have a secret hideout where they took Fluffs with them, but won't reveal its location until they get something in return. Give them 100 coins in exchange for the information.
Once at the lumber yard, look for a broken fence on the south side that you can squeeze under. Climb up the ladder in one of the buildings and you will find Fluffs. When you try to pick her up, she refuses to move, and also seems thirsty. Use the Bucket of milk on her. Try and pick her up again, but she stills refuse to move and is now hungry. Use the Doogle leaves on the raw sardine to make a Doogle sardine, which Fluffs will happily eat. Be sure to click through the entirety of the dialogue, or you'll have to give her another doogle sardine for the next prompt.
Try and pick her up again and she still refuses to move. The game will inform you that kittens can be heard nearby. Remember the jiggling crates you saw downstairs earlier? Climb down the ladder and search the jiggling crates until you find the one with three little kittens inside. The kittens will be in different crates for different players.
Go back to Fluffs and use the kittens with her. She'll run back home to Gertrude's house with the kittens (if you drop the kittens, they will run back to the same crate).
Return to Gertrude to claim your reward (a Pet kitten and some food).
Before Gertrude's Cat was updated, players could hop over a fence right next to the Sawmill operator's hut using a stile. Now, that has been replaced with a non-interactive gate and the entry point is to the west of the lumber yard.
In RuneScape Classic, the secret play area is different. Until RuneScape 2, the lumber yard was not accessible.
Also, after completing the quest you will receive the message in the chat box '... and some food' before the 'Quest Completed' message.
The 'jiggling crates' never used to jiggle, they would just have 'miow' popping up over them.
After completing the quest, if you talk to Juna, your character will say, "...I returned Fluffs safely to Gertrude, and then she gave me a cat of my own!" to which she replies, "Cats are one of the most mysterious creatures of Guthix. I hope you take your responsibility seriously."
Shilop and Wilough have t-shirts with the first letter of their name. This may be a reference to Fred and George Weasley from the Harry Potter series.