The Curiosity Shop's Back Room is a location featured in Majora's Mask.
The Back Room is accessible through a door at the Laundry Pool, in the south-west of Clock Town. However, it is locked during most times of the day. The only notable inhabitant of this area is Kafei, who uses the Back Room as his secret hideout.
The Back Room plays a part in the Anju and Kafei quest. After Kafei had the Sun's Mask stolen from him by Sakon, he could not bear to see his love, Anju without the mask. So the Man from Curiosity Shop kindly allows him to stay in the Back Room behind the Curiosity Shop (located in the Laundry Pool area) for as long as he likes.
Kafei spies on the customers of the Curiosity Shop, hoping to find Sakon among the patrons, so he can follow the thief back to his hideout. On the Night of the Second Day, Sakon drops by the shop and Kafei stalks after him. Link receives the Keaton Mask from the Man from Curiosity Shop and then, on the Final Day, him and Kafei begin their quest to take back the stolen mask from Sakon at Sakon's Hideout within Ikana Canyon.