It consists entirely of a presumably imaginary dialogue in a writer's head, between himself and an embodiment of the Muse Erato, after he wakes amnesiac in a hospital bed.

The New York Times called it "a surprisingly tedious novel," asserting that it was little more than Fowles' response to critics that he felt misunderstood his work.

"[2] Time magazine, in a more positive review, asserted that the book consists of a sort of intellectual play between Fowles and the reader, or by Fowles at the expense of all reading.

[3] The Pittsburgh Post-Gazette likewise identified the book as intellectually playful but found the dialogue tiresome.

[4] Dave Langford reviewed Mantissa for White Dwarf #55, and stated that "Although packed with allegory about what creativity/inspiration actually is, the book is wonderfully, unexpectedly funny.