Born the son of a patrician in the (fictional) 13th-century northern Italian town of Carmona, the world presents itself to Fosca as a mixture of violence and intrigue: While in the city the influential families fight for supremacy, this struggle is repeated in the outside world as a permanent state of war between the city-states and small states of Italy at the time and their ever-changing constellations of alliances.

Fosca gets the impression that these battles only go on endlessly because neither party has the time to permanently consolidate the power and rule it has won - and so the desire arises in him for a life that will last forever and thus give him the decisive advantage.

On the contrary, on a journey to the American colonies, Fosca is made vividly aware of all the misery of the inhabitants of this seemingly glamorous empire.

There, by chance, Fosca meets the adventurer Pierre Carlier, who succeeds in infecting him with his joy of discovery: The young man has set himself the ambitious goal of traveling to China and, on his way there, becoming the first European to cross the North American continent to the Pacific Ocean.

In the decadent circles of the nobility there, he first becomes a ruthless gambler who outplays all opponents and cannot be killed even in a duel - but this behavior does not provide him with lasting distraction.

As long as Marianne lives, Fosca clings to her, but he becomes increasingly aware of the insurmountable contrast with his fellow human beings, and becomes more and more indifferent to life.

When Fosca finishes his story, he tells that he suffers from nightmares in which the whole world is white and dead, populated only by two living beings: he and the mouse on which he tested the immortality potion.

The main tension exists between the meaninglessness of daily life, rituals, style from the perspective of an immortal man contrasted by the seeming trivial concerns of a mortal woman: the importance and the value they put on things are at opposite ends of the spectrum.

All Men are Mortal is identified as a "metaphysical novel" by Beauvoir's biographers Claude Francis and Fernande Gontier, who note that while it did not sell well, it was almost immediately translated into German.