The history of the city of Elovgrad is shrouded in secrets and legends. Previously, this territory was inhabited by an ancient people - the Lawgetts. There is a legend that in the 13th century. Knight William came here with his retinue and stood up to defend the Lowgetts from the rampaging warriors of the Order of the Holy Cross. The priests of the ancient people gave the knight a magic sword, with the help of which William defeated the crusaders. After which he disappeared without a trace. Neither the burial site nor his castle remains. Professor Volobuev, in search of traces of the legendary knight, meets the guys from the children's camp. Together they encounter mysterious events and unexplained phenomena.