Mert and Yusuf just finished seventh grade this summer, and they guide tourists in summer to contribute to their family budgets. Both of them learned three languages by just talking to tourists.
One day they come across with a tourist on Blue Mosque Square while searching tourists to guide. The tourist, Mr. Logan, asks the location of Basilica Cistern to them. He introduces himself as an archeologist to the children. A punk-looking guy meets with Mr. Logan. The children suspect that this guy might be a historical artifact smuggler.
After listening to these two men, they understand that the men will secretly enter into the tunnels of Hagia Sophia. The children are right about their suspicions. The guy who met Mr. Logan is a historical artifact smuggler and his nickname is “Marten.” The children think that if they complain to the police, the police won’t listen to them; therefore instead of directly going to the police, the children decide to follow them and find out what they are after.
The actual purpose of these two men is to find the secret reasures that were hidden in the tunnels of Istanbul. Mr. Logan is the head archeologist of one of the museums in London and specialized in manuscripts from ancient civilizations. The man with the nickname, Marten, will find the treasure using Mr. Logan’s expertise, and then he will sell it to Mr. Logan for a high price. Since the corridor beneath Hagia Sophia is the main junctions of the other tunnels, they must first enter from Hagia Sophia. While Sansar and Mr. Logan walk through the tunnels, the children follow them secretly. They find a message on the wall of the tunnel: “Go to Tardu if you are looking for the treasure!” To understand this message, firstly they must learn what Tardu means.
Children take a picture of the message and leave the tunnel. When they get out, they realize that they are near the Yenikapı walls. Mr. Logan and Marten walk a little further in the tunnels and find themselves in Üsküdar when they got out. Mert and Yusuf ask the meaning of the word they pictured to booksellers in used-book bazaar. They learn the meaning of Tardu, the light that comes from the dark. When Mr. Logan and Marten ask the same question to the same people, they learn that the two children asked the meaning of the word before they do. Mr. Logan understands that they are being followed and decides to go after the children. They find the children but the children manage to escape from them. Mr. Logan understands that, by Tardu, the Maiden’s Tower meant. Marten and Mr. Logan go to the Maiden’s Tower and enter the tunnel there. While trying to escape the men, the children enter the tunnels in Gülhane and get out of the Maiden’s Tower. They find a message in the tunnels of the Maiden’s Tower.
In this message, it is said that they must go to Tripoli to find the treasure. After searching the meaning of the message, Mr. Logan and Marten understand that they must go to the Hamidiye Clock Tower in Tripoli, Lebanon. While they are planning to go there, the children hear their plans. Mr. Logan and Marten take a touristic ship to Lebanon. Mert and Yusuf also take the same ship. The children put sleeping pills to drinks of Mr. Logan and Marten to make them. While they are sleeping on the ship, the children go to the Hamidiye Clock Tower in Tripoli. There they look at the pieces of the old clock and find a piece of paper with a message written in it, and they leave the tower. While the children are reading the message they just found, Marten and Mr. Logan see them and try to catch them. An officer, who saw that two men are chasing the children, calls the police. The police catch Mr. Logan and Marten. When the police read the note they found, the children realize that the real treasure is something very different from what they expected.