When the alchemist Cornelius Agrippa begins work on a secret elixir, his young apprentice Winzy believes it can cure him of only one thing: his hopeless love for Bertha. In a moment of despair, he drinks it, certain he will wake indifferent to her at last. He does not. Instead, he wakes more himself than ever, and Bertha, escaping the wealthy guardian who wants her married elsewhere, finally agrees to be his wife.
For years their happiness seems complete. Then Winzy notices what no one else will say aloud: while Bertha grows older, he does not change at all. Five years after that fateful night, Agrippa, dying, confesses what the elixir truly was, and the secret of Winzy's unchanging youth grows harder and harder to keep from the wife ageing beside him.
Narrated by Winzy himself at the close of more than three centuries of unasked-for life, The Mortal Immortal is Mary Wollstonecraft Shelley's quiet companion piece to Frankenstein: a story less interested in monsters than in marriages, and in what happens to ordinary human love when one partner is, by accident, no longer quite human at all.