Once in China, Sam meets Lu Min, the son of his dad's business client. They are the same age and have a lot in common. Lu Min tells Sam of the mystery of Chang Cheng (which is what the Chinese call the Great Wall). He says that tourists report hearing drumbeats coming from inside the wall. Sam and his family go with the Min's to visit the Great Wall. Being normal 11 year old boys, Lu Min and Sam split off from the group and end up falling through a trap door at the base of the wall and disappearing into another world. At first, they're not sure where they are or what is going on, but they know enough to stay quiet when they see two men coming out of the darkness, very feral men. They crawl to the direction the men come from and eventually come upon a city underneath and inside the great wall! The city is fascinating, complete with a castle. All the buildings are very narrow and tall to fit in the space. The animals are different as well as they have evolved in a different world.
While the boys have disappeared underground, Sam's father is throwing his weight around above ground and really being an ass. David Evans is going to have a lot to learn about how to handle people. The police close down that section of the wall completely and send all the tourists away to make the search easier. Chief Tao is in charge of the investigation and seems to be doing everything that he can, but David still isn't happy with it. David decides to go to the American Embassy to see if they can help. Unfortunately, the Embassy representative that he speaks to is only out for himself and looking for money. He calls the media to tip them off for a payout, which is what the Chinese government was trying to keep from happening. Sam's mom really isn't happy with all this. Her husband is going to have to learn to listen to her some more. Sam's mom and Mrs. Lu spend time together trying to piece things out.
While underground, a giant of a man named Tong runs across the two boys and he takes them to an alchemist named Yuan Chengang, a wallkeeper, who has lived there for over 2500 years. He tells Sam and Min that they are the chosen ones and will fulfill the prophecy. Chengang was originally an alchemist and advisor to a king who was sent to oversee construction of the wall. He had an apprentice alchemist, Wang Rente, who was very ambitious. Chengang was given 4 eternal cosmic silk worms, one red, one blue, one green, and one yellow, by a "small grey man with huge black eyes" (which sounds like an alien to me). He was to use them to help the people of the kingdom as they can do magic things when planted. Chengang was betrayed by Wang who stole the silkworms. Chengang recovered 2 of them and died after. He was buried underneath the great wall. He woke up in this kingdom which is inhabited by those who died serving the construction of the wall in some way. They choose their occupations there, so he is an alchemist again and is still looking for the two lost silkworms. His apprentice Wang is still there, but banned from the city because of greed. The prophecy says that these two boys will recover the lost worms. The boys agree to help him if he will help them escape when they are done. Chengang renames them Jin Tong (Sam) and Wei Lai Zhizi (Min) so that they will have names that fit for the city. The boys are exhausted after their day, so they just go to sleep and trust Chengang and his huge assistant, Tong. They learn that the mysterious drumbeats that people reported hearing outside the wall are a way of keeping time, since there is no sun to tell time by. Before long, the boys will run into the princess Lianhua and their trio is complete. The three of them will build a "tank" and go searching for the rest of the lost worms in the adventure of a lifetime.
The overall story of Chang Cheng: The Mystery Within is an interesting one, but the execution has quite a few flaws. Everything just seemed to happen without much buildup or lead in to what was going on. The conversations were extremely hard to read as the grammar is just plain wrong. I find it odd that Sam narrates what his parents are doing above ground while he is inside the wall, since there is no way he could know what was going on. They should have had one of the parents narrate. Of course, if this was all a dream, then that might explain more about how he knew what they were up to. I found it truly amazing (and unbelievable) that the boys would have been able to learn to shoot crossbows with deadly accuracy and build their own tank in just a few days. To be honest, the story felt more like something a boy would have dreamt about than something that would have actually happened. Maybe that's what it was supposed to be, a dream, but I don't think so. In total, Chang Cheng: The Mystery Within is an interesting read, but I'm not sure I would recommend it for most people. It is a fairytale that I can't promise will appeal to the majority.