The revised version of The Quest of the Holy Grail gives a greater role to Perceval, and introduces a number of knights not found in the Vulgate; but the largest change is that much of the story of Tristan (and of his rival Palamedes) is incorporated into the story.
The achievement of the Grail quest centres on Galahad's healing of Pellehan, which has to be accomplished before the knights can reach the Grail itself.
The Death of Arthur is little more than a relatively brief postscript, bringing the story of the adventures of the kingdom of Logres to an end; Lancelot and Guenevere are revealed as lovers, and Arthur fights both Lancelot and then the Romans.
Despite this victory, he is betrayed and killed by Mordred, as has been foreshadowed from the outset of the new material.
The romance ends with king Mark of Cornwall's death when he attempts to kill Lancelot and Bors at the hermitage to which they have retreated. For a full description of the Post-Vulgate Cycle see the blurb for the complete set.