Pyramus and Thisbe engage in a love that is forbidden because their families are feuding. They decide to take matters into their own hands and run away together. They agree that they will meet under the white berries of the mulberry tree at Ninus's tomb.
However, a lioness chases Thisbe, who drops her veil. Thisbe hides away in a cave. When Pyramus arrives, he sees Thisbe's veil, mangled and bitten by the lioness, and sees the lioness's tracks. He assumes Thisbe has been killed. He kills himself with his sword and blood from his arteries splurts up and stains the white berries of the mulberry tree red.
Thisbe, returning from the cave and finding Pyramus's body, kills herself too, but before dying, she prays to the mulberry tree that it keep its dark, purple-red color in remembrance of the lovers. The gods are moved by this plea, and the mulberry fruit remains red forever.
The transformation of the color of the berries symbolizes the way love has transformed the lovers so that they are willing to sacrifice their lives rather than live without one another. Red is a color associated with passion, and being in love transforms Pyramus and Thisbe so that they live more passionately—and also so that they are willing to die, another form of change. The red of the mulberries reinforces the theme of transformation.