In the land of Dagonaut a medieval squire named Tiuri and four other squires spend their last night before being knighted inside the chapel without chatting, sleeping, or eating. This is their last test in order to become a knight. But suddenly a badly wounded knight knocks on the door looking for help. Tiuri abandons the rules and opens the door. The knight asks him to deliver an important letter to king of Unauwen. The dying knight, Edwinem gives him a special ring as a proof that he was sent by the knight. During his journey, Tiuri passes through the land of Grey Knights where bandits rove. He loses his horse to the bandits and continues his travel without it but keeps the special ring that was given to him by Edwinem. In the Mistrinaut castle Tiuri is put in prison and is fighting with the so-called Grey Knights who suspect him for the murder of Edwinem, although the real culprits are Red Riders. The maiden Lavinia saves him by convincing her father, head of the castle, that Tiuri is innocent. Then, her father helps Tiuri on his mission by sending him some Grey Knights to accompany him on the next part of his journey. On the way, Tiuri meets someone who is on pilgrimage without knowing that he is actually a Red Rider spy. But when the Red Rider's life is in danger, Tiuri saves him, and the Red Rider has a change of heart. Then Tiuri goes to a hermit named Menaures. Menaures sends his boy Piak as Tiuri's guide. After traveling for a bit, Tiuri and Piak struggle with people who pose a threat to the safety of the letter, Tiuri opens the letter and memorises what is written on it. After that he burns the letter so that no one can read it. However, Tiuri does not know what the message on the letter means because the message is written in a different language. Piak helps him to remember it by making it into a song. Later, Tiuri and Piak have to cross the bridge over the Rainbow River have to cross the bridge but they have to pay a toll to do so. They don't have the money to pay for the toll, so they find an alternate route. Piak accidentally falls into the Rainbow River, but Tiuri saves him from drowning. They are then caught by a guard and they explain to him their mission. The special ring helps with their explanation when they show it to the operatives. Tiuri fortunately completes his mission, and delivers the letter to the king of Unauwen. The content of the letter is that the country is in danger because the enemy kingdom Eviellan have a plot against the country. Tiuri is honoured by the king for successfully bringing the message to him.
The film's plot does not include Marius, a forest dweller whom Tiuri first meets on his journey to Unauwen's realm, thereby forestalling a filmization of the original novel's sequel, in which Marius plays a pivotal role.
