Necessity begins forty years after these events, on the day the mortal form of the god Apollo, who had chosen to live as a human in the original city, finally dies, and he takes up his divine powers again.

Walton revisits familiar characters from her previous books (both Giovanni Pico della Mirandola and Socrates make an appearance), introduces some new ones and ties up loose ends.

In general, Necessity was received as "an adequately satisfying conclusion to [the] Thessaly trilogy,"[1] albeit with some reservations, specifically with regards to the description (or lack thereof) of the (re)contact between the Platonic civilization and wider humanity.

This was "initially touted as a major event," but "[r]ather than engage with the intriguing philosophical and ethical issues presented by this reconnection,"[1] the story focuses primarily on divine matters, and "the reader never gets to truly witness the culture clash [...].

It sees the plot as "mostly ... an excuse to explore whether or not Athene’s experiment is still working, discuss the nature of the soul, and engage in character development," and concludes the book is "[e]ngaging food for thought.