In Thomas Malory's original story included in his medieval Arthurian compilation Le Morte d'Arthur, Lynette (alternatively known as Linet, Linnet, Linette, Lynet, Lyonet) is a haughty noble lady who travels to King Arthur's court seeking help for her beautiful sister Lyonesse (also Linesse, Lioness, Lionesse, Lyones, Lyonorr, Lyonors), whose lands are besieged by the Red Knight. The young knight candidate Gareth picks up the quest, eventually marrying Lyonesse upon its completion, while Lynette becomes the lady of his brother Gaheris.
The two are sisters of Gringamore (Guinguemar) from Avalon. In Book IV: The Tale of Sir Gareth of Orkney, Dame Lynette comes to court asking for assistance against the Red Knight of the Red Lands. Since Lynette refuses to reveal her name for reasons which are not explained, she is presented with a kitchen servant instead of a champion. He says his name is Beaumains, but he is really King Arthur's nephew Gareth of Orkney in disguise. On their journey, the pair encounters the Black, Green, and Blue Knights, and finally the Red Knight of the Red Lands (Sir Ironside). Gareth slays the Black Knight, incorporates the others into Arthur's court, and rescues Lynette's sister Lyonesse. Lustily in love with Lyonesse, Gareth conspires to consummate their relationship before marrying. Only by the magical intervention of Lynette is their tryst unsuccessful, thus preserving Gareth's virginity and, presumably, his standing with God.
Gareth later counsels Lyonesse to report to King Arthur and pretend she does not know where he is; instead, he tells her to announce a tournament of his knights against the Round Table. This allows Gareth to disguise himself and win honor by defeating his brother knights. The heralds eventually acknowledge that he is Gareth right as he strikes down Gawain, his brother, in a joust. The book ends with Gareth rejoining his fellow knights and marrying Lyonesse in a joint wedding ceremony in which Gareth's other brother Gaheris marries Lynette, and their yet another brother Agravain marries Laurel, a niece of Lyonesse and Lynette.