This is Chrétien's fix-it fic for the story of Tristan and Iseut, a story he loathes because he is so squicked by adultery. Cligès, like Tristan, falls in love with his uncle's intended (and doesn't even have the excuse of a love potion to proffer for it). However, unlike the hapless King Mark, Cligès's uncle Alis is actually the one wronging his nephew by taking a bride, which he promised his dead, usurped older brother that he would never do so that his throne would pass to Cligès, who can therefore argue that his uncle started it.
Furthermore, rather than falling victim to a love potion, Cligès's resourceful sweetheart Fenice gets her nurse to brew up a potion of her own, which causes Alis to fall asleep every night in their marital bed and dry-hump his mattress whilst dreaming of his wife, meaning that their marriage is never actually consummated, meaning that it doesn't actually count, so there! I ... do not believe that it would work that way, but I haven't actually reviewed any relevant medieval case law, so I'll let Chrétian have his technical non-adultery.
Fenice, wanting to escape her sham marriage so she can finally get it on with Cligès, fakes her own death with the same stuff Juliet used. Unfortunately, she is intercepted on her way to her tomb by a lot of fancy Italian doctors, who take her pulse and notice that she is still, although unresponsive to their questioning. Finding this state of affairs unsatisfactory, said doctors proceed to whip her bloody, pour molten lead into her hands, and are finally preparing to grill her alive, apparently on the principle that if she isn't going to act like she's alive, she shouldn't be permitted to remain so any longer. At the crucial moment, however, a mob of indignant ladies intervene and deservedly defenestrate the doctors for having such terrible bedside manners.
Understandably, Fenice's convalescence from this ordeal takes time. She and Cligès live happily together for over a year in the secret tower that Cligès commissioned from the same serf who built the trick crypt for Fenice. It has a fancy garden and everything and they would apparently stay there happily forever if one of Alis's men didn't climb into the tower garden to retrieve a mislaid hawk and surprise them in flagrante
. This man is so credulous he is actually prepared to believe that the woman that Cligès is getting it on with just happens to look exactly like the late lamented empress, but Cligès gives it all away by saying Oh noes, now I have to kill this guy to protect our secret,
so then he does have to kill that guy to protect their secret. Except, in spite of pwning all of King Arthur's knights four days running in different colored suits of armor earlier, he only manages to lop off this poor guy's leg, so he drags himself back to Alis to break the bad news to him.
Cligès gets Arthur to send him an army to fight his uncle, but conveniently by the time he gets back, Alis has died of grief, so he can get married and assume his throne without any casualties besides Missing Leg Guy. And the moral of the story is: you can still live happily ever after if you weasel out of your technical adultery.