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.

It is a beautiful summer night, and the last echoes of the hunting horn are dying away on the evening breeze, when Ysolde turns to Brangeane, and impatiently bids her put out the light.  The terrified nurse refuses to do so, and implores Ysolde not to summon her lover, declaring that she is sure that Melot, one of the king’s courtiers, noted her pallor and Tristan’s strange embarrassment.  In vain she adds that she knows his suspicions have been aroused, and that he is keeping close watch over them both to denounce them should they do anything amiss.  Ysolde refuses to believe her.

The princess is so happy that she makes fun of her attendant’s forebodings, and, after praising the tender passion she feels, she again bids her put out the light.  As Brangeane will not obey this command, Ysolde, too much in love to wait any longer, finally extinguishes the light with her own hand, and bids her nurse go up in the watch-tower and keep a sharp lookout.

Ysolde then hastens to the open door, and gazes anxiously out into the twilighted forest, frantically waving her veil to hasten the coming of her lover, and runs to meet and embrace him when at last he appears.