The sight of the medicine chest in which it is secreted unfortunately reminds Ysolde that she too knows the secret of brewing draughts of all kinds, so she prepares a deadly potion, trying all the while to make Brangeane believe that it is a perfectly harmless drug, which will merely make her forget the unhappy past.
While she is thus occupied, Kurvenal suddenly appears to announce that they are about to land, and to bid her prepare to meet the king, who has seen their coming and is wending his way down to the shore to bid her welcome. Ysolde haughtily replies that she will not stir a step until Tristan proffers an apology for his rude behaviour and obeys her summons. After conferring together for a few moments, Tristan and Kurvenal agree that it will be wiser to appease the irate beauty by yielding to her wishes, than to have an esclandre, and Tristan prepares to appear before her. Ysolde, in the mean while, has passionately flung herself into Brangeane's arms, fondly bidding her farewell, and telling her to have the magic draught she has prepared all ready to give to Tristan, with whom she means to drink atonement.
While Brangeane, who mistrusts her young mistress, is still pleading with her to forget the past, Tristan respectfully approaches the princess, and when she haughtily reproves him for slighting her commands, he informs her, with much dignity, that he deemed it his duty to keep his distance:—
‘Good breeding taught,
Where I was upbrought,
That he who brings
The bride to her lord
Should stay afar from his trust.’
Ysolde retorts, that, as he is such a rigid observer of etiquette, it would best behoove him to remember that as yet he has not even proffered the usual atonement for shedding the blood of her kin, and that his life is therefore at her disposal. Tristan, seeing she is bent upon revenge, haughtily hands her his sword, telling her that, since Morold was so dear to her, she had better avenge him. Under pretext that King Mark might resent such treatment of his nephew and ambassador, Ysolde refuses to take advantage of his defencelessness, and declares she will consider herself satisfied if he will only pledge her in the usual cup of atonement, which she motions to Brangeane to bring.
The bewildered handmaiden hastily pours a drug into the cup. This she tremblingly brings to her mistress, who, hearing the vessel grate on the pebbly shore, tells Tristan his loathsome task will soon be over, and that he will soon be able to relinquish her to the care of his uncle.
Tristan, suspecting that the contents of the cup are poisonous, nevertheless calmly takes it from her hand and puts it to his lips. But ere he has drunk half the potion, Ysolde snatches it from his grasp and greedily drains the rest. Instead of the ice-cold chill of death which they both expected, Tristan and Ysolde suddenly feel the electric tingle of love rushing madly through all their veins, and, forgetting all else, fall into each other's arms, exchanging passionate vows of undying love.
Brangeane, the only witness of this scene, views with terror the effect of her subterfuge, for, fearing lest her mistress should injure Tristan or herself, she had hastily substituted the love potion intrusted to her care for the poison Ysolde had prepared. While the lovers, clasped in each other's arms, unite in a duet of passionate love, the vessel is made fast to the shore, where the royal bridegroom is waiting, and it is only when Brangeane throws the royal mantle over Ysolde's shoulders, and when Kurvenal bids them step ashore, that the lovers suddenly realise that their brief dream of love is over.
The sudden revulsion from great joy to overwhelming despair proves too much for Ysolde's delicate frame, and she sinks fainting to the deck, just as King Mark appears and the curtain falls upon the first act.
Several days are supposed to have elapsed, when the second act begins. Ysolde after her fainting fit has been conveyed to the king's palace, where she is to dwell alone until her marriage takes place, and where she forgets everything except the passion which she feels for Tristan, who now shares all her feelings. In a hurried private interview the lovers have arranged a code of signals, and it is agreed that as soon as the light in Ysolde's window is extinguished her lover will join her as speedily as possible.