Ronny and his friends live in Selby Road. One day he shows them a present he got from his aunt. It is a spy kit and is full of disguises, invisible ink and codes. At first they have fun together as they try on the disguises and work out the codes. Grosmont House is a big, empty house near Selby Road, and the children sometimes play there. One night, Kate sees flashing lights coming from the house. Why is somebody in the house? Is this a code? The Master Spies of Selby Road quickly work together to find out. And they find a lot more than just the answer...