"Run to the shop for me," Rory's mother said. But when Rory arrived he found only a notice which said BACK IN HALF AN HOUR. The door was locked.
"Ah, but I know where Mr. Marsden keeps the key," Kay said. Kay was Rory's friend. She knew about things like secret keys.
The key worked, and soon they were inside the shop. That was exciting enough. But it was only the start of the adventure.