In the Vulgate Merlin Queen Guinevere had a half sister, who was also called Guinevere, commonly called the False Guinevere. They were born on the same day and looked exactly the same, though they had a different mother. King Leodegrance of Camelide had slept with his seneschal's wife.
While Lancelot was staying with his friend Galehaut in Sorelois, the False Guinevere plotted with Bertholai, a knight of Camelide, to convince Arthur that she was his real wife. The False Guinevere said in her letter that if the king would not take her back as his wife, Arthur would have to return the Round Table to her, since it was originally a wedding gift from her father.
The ring that the False Guinevere had was identical to that of the Queen. Arthur set Boxing Day as the day to prove which was his real wife and which was the impostor. Twenty barons from Camelide were part of the conspiracy.
One day, Bertholai captured Arthur in the forest. They gave Arthur a love potion so that he would fall in love with the False Guinevere. Arthur declared his real wife as an impostor, and wanted to have her executed. Arthur with the barons of Camelide found Guinevere guilty of treason.
The Knights of the Round Table, particularly Gawain, were shocked at the accusation and verdict. Gawain and the knights opposed Arthur and the conspirators; saying they would rather that he banish them than allow the queen to be exiled or put to death.
News arrived about Guinevere's imprisonment. Lancelot immediately set out to rescue the queen, accompanied by Galehaut and his army.
When the day arrived that Guinevere was to be executed, Lancelot challenged the barons of Camelide in the force of arms. Bertholai declared that Lancelot would have to fight the three strongest knights of Camelide, instead of one. Arthur ordered Lancelot to back down but Lancelot refused, then renounced his allegiance to Arthur and his seat at the Round Table.
Lancelot fought the three knights, killing two and wounding the third knight. Though, Lancelot had won the combat, Arthur refused to take the real Guinevere back (because Arthur was still under the spell of the False Guinevere).
Months later, the False Guinevere and Bertholai fell mysteriously ill, robbing them of their ability to move. They realized God was punishing them for the plot to remove the real queen. To save their souls, they repented of their sins and crime to Arthur's chaplain, Amustans and also confessed to Arthur of their deception.
When news arrived of the conspirators' deaths, they also found that Arthur wished to take his wife back. At first Guinevere pretended that she would not return to Arthur since he was ready to execute her. Arthur had to reconcile with Guinevere, Lancelot and Galehaut. Arthur pleaded with Lancelot to retake his seat at the Round Table. Once Arthur and Guinevere reconciled, the queen gave a kiss to Lancelot in public for his service to her, and she declared Lancelot as her personal champion, which Arthur approved.