Wan Tang, a second-hand bookstore owner, was sorting through old books on a rainy night when she discovered an unfamiliar handwriting on the fourth page of a 1937 thread-bound diary. She wrote down her questions with rainwater, and received a reply from Hengchuan, a watchmaker in the Republic of China. From then on, every rainy day, the two exchanged information through the pages of the book: she described the convenience of mobile payment, and he drew a diagram of the gears of an old-fashioned clock. When Hengchuan mentioned the "fire in the watch shop on a rainy night in July" recorded in his diary, Wan Tang was shocked to realize that this was exactly the same as the fire news clippings left by his grandmother. They must let Hengchuan find the arsonist before the rainy season ends, and Wan Tang must decipher the page keeper's code hidden in the diary, otherwise the connection across time and space will be forever broken.
