In Felix and the Sword of Sefu, twelve-year-old Felix Snow has to rescue his little brother, Ethan, when the two boys and their older sister, Sadie, are accidentally transported into a magical world. Kimaji is a country where magic exists, but has been illegal for a long time. A young native of Kimaji named Flora has been searching for her missing sister, Clio, and has cast a forbidden spell to find her. The spell fails to locate Clio, but does pull Felix, Sadie, and Ethan out of their world and into Kimaji. Flora tries furiously to send the three of them home, but before she is able to do so, Ethan is kidnapped by a troll. It soon transpires that the troll is very likely to be an evil sorcerer, known as Silk, in disguise. Silk is a master of illusion and has been holding the country ransom for a legendary sword – the sword of King Sefu.

Armed only with an enchanted bag of coins and an ancient riddle that points to the location of Sefu's sword, Felix sets out to find the sword and use it to fight the sorcerer for the return of his little brother. If he doesn't succeed in finding the sword and defeating Silk, he, Sadie, and Ethan may be lost forever.