The action centers on Lancelot's rescue of the queen after she has been abducted by Meleagant, the son of Bademagu. The abduction of Guinevere is one of the oldest motifs in Arthurian legend, appearing also in Caradoc of Llancarfan's Life of Gildas, and carved on the archivolt in Modena Cathedral. After Chrétien's version became popular, it was incorporated into the Lancelot-Grail Cycle and eventually into Thomas Malory's Le Morte d'Arthur. The story deals with Lancelot's trials rescuing Guinevere, and his struggles to balance his duties both as a warrior and a lover bound by the conventions of courtly love.
Lancelot, The Knight of the Cart, starts off with Guinevere being abducted by Meleagant, after his tricking Arthur into allowing him to do so. After Gawain protests Arthur’s decision to let them go, Arthur obliges to let Gawain go after them. While Gawain is looking for the pair, he runs into the then unnamed Lancelot who, after completely wearing out his horse to death, convinces Gawain to lend him a horse and he then speeds after Guinevere. When Gawain catches up to him, Lancelot has worn out his new horse to death just as he did his previous one.
Lancelot winds up confronting a cart-driving dwarf about telling him which way Guinevere and her captor went. The dwarf tells him that he has to ride in his cart if he wants to find out where the two went, and Lancelot reluctantly agrees even though this places him in a humiliating position. Gawain, not about to emasculate himself, chooses to follow them on horseback. Along this journey they encounter many obstacles, with the most prominent one coming from other people being unwilling to talk to Lancelot because of his implied low status because of the cart, something that affects him throughout. His first trial comes when a maiden offers a bed for the knights, but refuses to let Lancelot lay on it. It is then revealed to be a trap to kill the knights, but it does not faze Lancelot.
After many more encounters with beautiful women and rude knights, Lancelot and Gawain decide to part so that they may cover more ground. Lancelot finds Guinevere in the castle of Gorre, but is then driven away by her coldness, which is later revealed to be due to his initial hesitation to enter the cart. Lancelot leaves to find Gawain but is then drawn back and Guinevere apologizes to him.
They spend a passionate night together after Lancelot breaks into her tower. He injures his hand during his break-in, and leaves blood all over Guinevere's sheets. Lancelot sneaks out of the tower before sunrise, and Meleagant accuses Guinevere of committing adultery with Kay, who is the only wounded knight nearby. Lancelot challenges Meleagant to a fight to defend Guinevere’s honor.
After Meleagant’s father interferes, Meleagant and Lancelot then agree to fight in a year's time. Over this year's time, Lancelot is tricked by another dwarf and forced into imprisonment while Guinevere is allowed to return home. When it comes time to duel, Lancelot bargains with his captors to let him go and fight, and that he promises to return.
When he finally did fight the tournament fighters, Guinevere asks for him to lose to prove his love. He obliges and when he starts to lose, Guinevere changes her proposal, now hoping for him to win. Lancelot complies and beats the other tournament competitors, and afterwards he returns to his captors.
Meleagant finds out from the captor's husband, that his wife was the one who agreed to release Lancelot temporarily (to fight at the tournament). Meleagant orders Lancelot to be locked away in a master craftsman's castle. de Troyes ends his section with Lancelot imprisoned.
Godfrey of Lagny continues the story from here on out: The woman whom Lancelot saved from kidnapping (she ordered Lancelot to sever her stalker's head), was actually Meleagant's sister; Meleagant's sister searches for Lancelot in order to return his favor. She finds an axe, and the rope used by Lancelot to pull up food, pulls up the axe instead. Lancelot hammers his way out and escapes with her to a secluded home that she owns.
Meanwhile Gawain prepares to battle Meleagant, since Lancelot is missing (a one year rematch after the second duel, was established). Lancelot arrives on time and fights Meleagant, who loses his temper and arm, and then is beheaded by Lancelot. Guinevere embraces Lancelot restrictively (they are in public) in the end.