Back at the office, the captain asks if there's anything special about the tracking matrix. It's just a way to track the back flow of documents. The captain says he once saw a group of businessmen use this tracking matrix on their domestic laser printers, so maybe they can use it to track backflow of documents as well. What's the magic trick? A few days later, the elderly couple tells the captain that baokang has returned home with an old friend. The old friend gave him the money, but he hasn't spoken to his family since. The elderly couple doesn't know what to do, so the captain tells them to hurry up and get some fish in their lunch. He's got some news: the analysis results showed that little wu's and gao's books were printed on the exact same machine. They should arrest the two men and take them to the police station.