In "Transformation," Guido tells the story of how, when he was a young man, he squandered his family fortune, forcing his family friend Torella to cancel the marriage contract between Guido and Torella's daughter, Juliet. When Guido's attempts to kidnap Juliet are thwarted, Guido is banished from his home city of Genoa. As Guido wanders the beach in despair, he witnesses a shipwreck and sees a strange, misshapen creature emerge out of the sea. The creature offers Guido a some treasure if he will trade bodies with him for three days. Guido agrees, and when the creature does not return after the third day, Guido returns to Genoa in the creature's body. As he feared, the creature (in Guido's body) has reconciled with Torella and is about to marry Juliet. Guido attacks the creature and kills it, but is wounded himself and loses consciousness. He awakens in his own body and makes an almost complete recovery, and he and Juliet marry.
Of all the tales in this edition, "Transformation" seems closest to Frankenstein in its engagement with the motif of doubling, its use of the supernatural, and its tale-within-a-tale structure.
The illustration is an engraving after a painting of Juliet from Shakespeare's Romeo and Juliet, but the tale re-imagines the figure as the Juliet of the tale.
For more information about "Transformation," visit its Wikipedia page. An HTML version of the tale is also available from Columbia University.