Tristram and Larnerok meet again by accident, this time in the Perilous Forest of North Wales. They unwittingly fight each other, then again swear friendship. They meet Palomydes, who unhorses them both and rides on in pursuit of his beast. Now Tristram and Lamerok part and undergo separate adventures, many of them involving single combat with Round Table knights.
Arthur is in the Perilous Forest through the craft of a sorceress, Aunowre, who captured him to make him her lover and, finding he will not be faithless to Guinevere, now hopes to destroy him by sending him, without his identifying arms, against all the knights who, to increase their glory, fight any who happen to pass. Arthur's knights, here to hunt for him, are unwittingly his most dangerous enemies.
Tristram meanwhile reaches Cornwall and Isode, discovers love letters that have passed between her and another knight, and goes mad. He runs naked for a time, then recovers and is exiled by Mark. In exile he encounters the Round Table knights who are looking for Arthur. He beats many of them, helps Launcelot when his life is threatened by Morgan le Fay, helps Gawain, and enters a tournament near the Castle of Maidens.
At the tournament he tells no one his name — fights under the title "The Knight with the Black Shield — and on the first two days departs secretly after he has won the field. On the third day he beats and shames his old enemy Palomydes, fights Arthur himself to a standstill, then is wounded almost mortally by an accidental stroke from Launcelot. He flees to a forest; Palomydes follows him to kill him; but despite his wound, Tristram beats Palomydes, then Gawain's brother Gaheris. Launcelot, still at the tournament, is declared winner of the field but gives the prize to Tristram as more deserving.
Ten of Arthur's best men go to find Tristram. Launcelot, riding in the wrong direction, rescues a damsel; Lucan and Ywain unwittingly fight with Tristram and are beaten. Tristram is finally found not by Arthur's men, but by an enemy — the father of a man he killed in the tournament — and is thrown in prison along with Sir Dynadin and Palomydes. Tristram falls sick.