The story of Yvain beings because of an encounter Yvain's cousin Calogrenant once had at a magic spring. The water of the spring creates storms when it's poured onto the ground. Calogrenant says,
The storm was so fierce and terrible that a hundred times I thought I should be killed by the bolts which fell about me and by the trees which were rent apart.
This quote emphasizes the power of the storm created by the magic water, which comes up several times in the story.
When Yvain asks his new wife, Lady Laudine, if he can go with King Arthur and his group to participate in the tournament, she says,
I grant you leave until a certain date; but be sure that my love will change to hate if you stay beyond the term that I shall fix. Remember that I shall keep my word; if you break your word I will keep mine. If you wish to possess my love, and if you have any regard for me, remember to come back again at the latest a year from the present date a week after St. John's day; for today is the eighth day since that feast. You will be checkmated of my love if you are not restored to me on that day.
This is important because it sets up the rest of the story. Yvain forgets to return to Lady Laudine, which causes him great heartache later in the story.
When Lady Laudine's envoy arrives, she says,
But Yvain has caused my lady's death, for she supposed that he would guard her heart for her, and would bring it back again before the year elapsed. Yvain, thou wast of short memory when thou couldst not remember to return to thy mistress within a year. She gave thee thy liberty until St. John's day, and thou settest so little store by her that never since has a thought of her crossed thy mind. My lady had marked every day in her chamber, as the seasons passed: for when one is in love, one is ill at ease and cannot get any restful sleep, but all night long must needs count and reckon up the days as they come and go. Dost thou know how lovers spend their time? They keep count of the time and the season. Her complaint is not presented prematurely or without cause, and I am not accusing him in any way, but I simply say that we have been betrayed by him who married my lady. Yvain, my mistress has no further care for thee, but sends thee word by me never to come back to her, and no longer to keep her ring. She bids thee send it back to her by me, whom thou seest present here. Surrender it now, as thou art bound to do.
The envoy accuses Yvain of being a treacherous liar instead of a noble and honest knight; nobility and honesty are qualities prized by King Arthur's court. The accusations demolish him so that he's incapable of speech and runs away to live in the woods. He stays there until a noblewoman saves him—and he embarks on a series of quests to regain his honor.
When Yvain saves a lion from a poisonous, fire-breathing serpent, he expects the lion to attack him. It doesn't; instead, it shows gratitude. De Troyes writes,
And the lion walks close by his side, unwilling henceforth to part from him: he will always in future accompany him, eager to serve and protect him.
If Yvain had not stopped to free the lion, he would have fallen in the fight with the sons of evil later in the story. He also takes the lion as part of his image, telling people to refer to him as "the Knight with the Lion."
Yvain shows he's learned from his mistakes when he says to Lady Laudine at the end,
Lady, one ought to have mercy on a sinner. I have had to pay, and dearly to pay, for my mad act. It was madness that made me stay away, and I now admit my guilt and sin. I have been bold, indeed, in daring to present myself to you; but if you will deign to keep me now, I never again shall do you any wrong.
(The entire section is 1,529 words.)