Blissful in each other’s company, Tristan and Ysolde now forget all else, while they exchange passionate vows and declarations of love, bewailing the length of the days which keep them apart, and the shortness of the nights during which they can see each other.  In a passionate duet of mutual love and admiration, they also rejoice that, instead of dying together, as Ysolde had planned, they are still able to live and love.

Brangeane, posted in the watch-tower above, repeatedly warns them that they had better part, but her wise advice proves useless, and it is only when she utters a loud cry of alarm that Tristan and Ysolde start apart.  Simultaneously almost with Brangeane’s cry, Kurvenal rushes upon the scene with drawn sword, imploring his master to fly; but ere this advice can be followed King Mark and the traitor Melot appear, closely followed by all the royal hunting party.  Ysolde, overcome with shame at being thus detected with her lover, sinks fainting to the ground, while Tristan, wishing to shield her as much as possible from the scornful glances of these men, stands in front of her with his mantle outspread.  He, too, is overwhelmed with shame, and silently bows his head when his uncle bitterly reproves him for betraying him, and robbing him of the bride he had already learned to love.  Even the sentence of banishment pronounced upon him seems none too severe, and Tristan, almost broken-hearted at the sight of his uncle’s grief, sadly turns to ask Ysolde whether she will share his lot.  Shame and discovery have in no wise diminished her affection for him, and when she promises to follow him even to the end of the earth he cannot restrain his joy, and notwithstanding the king’s presence he passionately clasps her in his arms: 

   ’Wherever Tristan’s home may be,
    That will Ysolde share with thee: 
    That she may follow
    And to thee hold,
    The way now shown to Ysold’!’

Melot, enraged at this sight, rushes upon Tristan with drawn sword, and wounds him so sorely that he falls back unconscious in Kurvenal’s arms, while Ysolde, clinging to him, faints away as the curtain falls on the second act.

The third act is played in Tristan’s ancestral home in Brittany, whither he has been conveyed by Kurvenal, who vainly tries to nurse his wounded master back to health and strength.  The sick man is lying under a great linden tree, in death-like lethargy, while Kurvenal anxiously watches for the vessel which he trusts will bring Ysolde from Cornwall.  She alone can cure his master’s grievous wound, and her presence only can woo him back from the grave into which he seems rapidly sinking.