This is the first novel where Lancelot and Guinevere have a romantic relationship with each other, but to reconcile this adultery, Chrétien placed the characters in a setting where this was not really considered "cheating"
One day during dinner a strange Knight bursts into castle and announces he has prisoners of King Arthur's land. He will not release them unless a knight rides into the forest with Guinevere to protect her. Sir Kay threatens to leave if Arthur does not let him protect Guinevere, so he sends his wife out with Kay as her protector. Before leaving, the Queen says if a certain knight where there he would never let Kay be her protector. Eventually Sir Gawain gets worried about his aunt, and the court rides out to find them. They only find Sir Kay's horse with a bloody saddle.
A mysterious knight rides up and asks Gawain for his extra horse, and then rides off after Guinevere. After asking Arthur's permission, Gawain follows the other knight, who we later find out is Lancelot. Lancelot rides his second horse to death and is walking when a dwarf driving a cart drives past. Lancelot asks the dwarf for news of Guinevere, and the dwarf refuses to give any information unless Lancelot rides in the cart. He jumps in, and Gawain catches up later and follows the cart. The cart passes through a town where Lancelot is mocked and teased, and Lancelot is offered a place in the home of the local lady's maid's house. The next morning they catch sight of Guinevere but they fail to catch them. During the chase, they encounter in the woods a maid who informs them that the captor is Meleagant, a king's son, and that they live in a realm with only two entrances, and from which you cannot return. The method that Lancelot takes is the Sword Bridge, which is an elongated sword. Gawain follows on the other route, the Water Bridge, which is a bridge covered in water. The only stop Lancelot makes on the way to the sword bridge is at a church where he finds tombs with the names of the Knights of the Round Table on them. One tomb in particular takes seven men to lift, and would allow the release of prisoners in Meleagant's realm. Lancelot's love for Guinevere fuels a superhuman strength that allows him to lift it.Lancelot finally reaches the Sword Bridge. He removes his gloves and boots and crawls across the bridge, even though he is left terribly wounded. King Bademagu's (Meleagant's father's) castle is right up the road. The king and his son have been watching, and the king urges his son to return Guinevere without fighting, because he knows Lancelot is the better fighter. Meleagant refuses, so Lancelot demands to fight. The king begrudgingly agrees and sets a tournament for the following day.
The next day the men fight and Guinevere watches them from a tower. Lancelot's wounds start to hurt him again, and he begins to lose ground. A maid suggests that Guinevere cheer him on, and this is the first time they say Lancelots name. He gets distracted and Meleagant attacks. Embarrassed, Lancelot forces Meleagant between himself and Guinevere so that he can fight and gain strength from her cheering. The king begs her to stop them from fighting, and she asks Lancelot to stop. He stops fighting right away, and Meleagant takes advantage of this and attacks him anyway. Ashamed of his son's dishonor, the King sets new conditions to the fighting: Lancelot and Kay will heal there and then return to Arthur's court with Guinevere. When he is ready, Meleagant will come to challenge him. A year later they will fight.
Lancelot is taken to Guinevere, but she is upset with him. He visits Kay and then goes to find Gawain. He is captured on the way. After hearing that Guinevere is dead, he gets back to the castle, and finds her alive. She apologizes for being upset, and they spend all night talking. Later that night he sneaks out to talk to her across the grille in front of her window. At her request, he bends open the bars of her cell to spend the night with her. The next day he goes to search for Gawain again, but is tricked by a dwarf into being separated from his friends. They save Gawain from the water bridge and then find a letter saying Lancelot went home. They return to Arthur's court but no one has heard from Lancelot.
A tournament is held, and Lancelot, hearing that Guinevere will be attending it, convinces his captor (a maid paid by Meleagant to watch him) to let him attend the tournament and return right after. She lets him, and Lancelot wins the tournament and then returns to his prison. Meleagant, who is upset that his prison didn't keep Lancelot, builds a tower near Gorre and locks Lancelot inside, sealing everything except a small window where some food is passed through. While Lancelot is locked away, Meleagant goes to King Arthur's court to challenge Lancelot.
Gawain says he will fight if Lancelot cannot be found in a year, and despite everyone searching, no one can find him. Then, Meleagant's sister finds Lancelot in the tower, and nurses him back to help. He returns to King Arthur just as Gawain is getting ready to fight Meleagant. Lancelot beats Meleagant, saving Guinevere and strengthening their love.
1.The Knight of the Cart by Chrétien de Troyes
3.2. A prose translation of the poem