What do Palamides and King Mark attack as part of a larger plot to overthrow King Arthur and take over the country?
The blacksmith and swordsmith John is tutored at the court of King Arthur, but as a commoner he can't hope to win the hand of Lady Linet, daughter of the Earl of Yeonil. The Earl's castle is attacked by Saracens and Cornishmen — disguised as Vikings — and his wife is killed, making him lose his memory. The attack was part of a plot by the Saracen Sir Palamides and the pagan Cornish King Mark to overthrow Arthur and Christianity and take over the country, whilst pretending to be Arthur's friends and allies  - Palamides is a knight of the round table and Mark has faked his own baptism.John accuses Palamides' servant Bernard of murder before Arthur, who grants him three months' grace to prove the accusation or face execution himself. Another knight, Sir Ontzlake, takes pity on John and trains him in swordplay so that he can take on an alternative secret identity as the wandering Black Knight. The "Vikings" raid a newly founded monastery and take Lady Linet and its monks to Stonehenge for a pagan sacrifice, but the Black Knight arrives and saves her, closely followed by Arthur and his knights, who defeat the pagans and destroy Stonehenge.
Sir Palamides tricks the Lady Linet into his castle to try to get her to reveal the Black Knight's identity, but John is informed of this and saves her, still in disguise. Sir Ontzlake then sends him to King Mark's castle, where a pro-Arthur woodcarver shows him  a secret tunnel into the royal chambers. John arrives in time to overhear Mark and Palamides finalising their plot but Palamides beats him back to Camelot, tricking Arthur into thinking that the Black Knight is leading the Viking raids. John arrives dressed as the Black Knight and despite revealing his identity is briefly imprisoned until Lady Linet and Sir Ontzlake free him, with the latter standing bail for John to Arthur.
The Earl's castle