The Arthurian film cycle continues with this Adventures of Sir Galahad serial. In this serial, the youth Galahad (George Reeves), trying to emulate his father Sir Lancelot, wants fervently to be admitted to the Knights of the Round Table order. When he defeats Sir Bors and Sir Mordred in tournament, King Arthur agrees to knighthood if he can guard Excalibur for one night. Unfortunately, during that night the sword is stolen by a mysterious personage known only as the Black Knight. Possession of Excalibur makes the holder invincible and without it the sovereignty of Arthur is endangered. Galahad is refused admission to King Arthur's Round Table until he regains the missing magic sword Excalibur. Ulric, the Saxon King, invades England. Galahad, aided by Sir Bors, attempts to retrieve Excalibur from Bartog, Ulric's chief aide, who has gotten it from the mysterious Black Knight. Merlin the Magician harasses Galahad at every turn, while Morgan le Fey, Arthur's sister and also a magician, helps Galahad fight both Merlin's magic and the Saxons. The Black Knight, a traitor within Camelot, is conspiring with the Saxons and a band of outlaws to overthrow King Arthur. When Queen Guinevere is seized by the traitors, Merlin relents and sends Galahad to the Lady of the Lake, who gives him Excalibur.