Driskoll is helping the local bard, an extremely obese human named Oswald, remove the evil magic from a bunch of scrolls recovered from the ruins near Curston, but he can't help but try to look at them, even if the scrolls could be cursed and visit some bad magical effects on him. One scroll in particular catches his eye, one that has a dagger drawn on it. To Driskoll's horror, the dagger has Kellach's name on it!
Oswald tells Driskoll that the dagger is a magical item known as the Dagger of Doom. Two brothers, Cor and Adrian, lived in Curston when it was still called Promise. Cor grew up to become a Knight, and Adrian, a mage. Cor became a Knight of the Silver Dragon, and Adrian did not, and the rejection festered inside him. When Cor realized Adrian's feelings, he presented his brother with the magical dagger. Adrian looked on the dagger as a present out of pity and grew to hate his brother more. He flew into a rage and stabbed his brother with the dagger and fled. Cor's friend, a local monster named a mantlehorn, found Cor and pulled the dagger from his breast. Everyone could see his name blazoned on the blade. The mantlehorn took Cor away and buried him with the dagger. From then on, the Dagger became known as a device that caused strife between brothers and would cause one to kill the other.
Driskoll is shaken by the story, and hurries home to tell Kellach of his vision. Along the way, he runs into Moyra and she shows him Kellach's new secret: there is a girl he is interested in and seems to be trying to woo. When Driskoll tells Kellach the story of the dagger and tries to warn him, Kellach dismisses his concerns.
They find their Dad distracted. Someone is attempting to build a castle near Curston. Actually, rebuild would be a better word. But there is no Castle near Curston, is there? They actually find a ruined castle that is protected by magic to be invisible unless you find it in a specific way. While they are checking out the ruins, Driskoll and Kellach end up in a pit, and Kellach thinks Driskoll tipped him into the pit deliberately. They manage to make up and get out together, but before they leave, they find an old man named Hadrian, who is the one rebuilding the castle. Also, someone throws a Dagger at Kellach, a knight dressed in armor that seems to be made of shards of mirrors. The dagger is the same one as pictured on the scroll, and Kellach once again thinks that Driskoll is the one who threw the dagger.
The next day, the boys anger their father by returning to the site of the castle, which he thinks is dangerous. The overseer, a man named Scraper, offers their father, Torin, a task to keep them occupied. Help rebuild the keep for a few days. Torin agrees, and Driskoll learns from the Dwarven stonemasons that they have been looking for the Dagger to destroy it, because of its evil reputation.
Kellach, who has been working with Hadrian, returns and shows Driskoll how the knife can cut through stone like a hot knife through butter. Later, Driskoll goes back to Oswald to try to learn more about the knife, but he has been poisoned by some ale that was sold to him by a man in a black cloak... a man who they know as Scraper.
Driskoll and Moyra try to enlist the aid of Kellach, who still thinks that Driskoll is behind all the attacks and strangeness they have been experiencing the past few days. Driskoll tries to decipher the runes on the mirror blade and comes up with Evil Renwo. Seeing that the girl Kellach has been trying to impress is Willeona Renwood, he and Moyra think that she has something to do with the dagger.
But when they return to Promise castle to confront Scraper and Willeona, Driskoll and Kellach are attacked by the mirrored knight. Everytime they argue or accuse one another of doing something bad, the Mirror Knight gets stronger. Can they put aside their arguments long enough to defeat the Knight, or one of them is forced to kill the other?
I like all of the Knights of the Silver Dragon Stories, but I found this one to be the best so far. While there was an external enemy, the Mirrored Knight, most of the conflict in the story was between the two brothers, in the way that sibling conflicts can sometimes get out of hand. The mystery at the story's heart was extremely well done, and left even me guessing as to the eventual outcome and the real villain.
The story also ramped up the tension with the arguments and bad feeling between the brothers. Every time they argued or got angry with one another, readers have to wonder, "is this because of the dagger or a normal sibling disagreement?" As I said, I consider this the best book so far, as it is very mature in the nature of the mystery and the final solution. If you only read one book from this series, let this one be it!