Omnivore has as its frame the investigation of the deaths of eighteen travelers from Earth to the distant planet Nacre.

The planet is notable for its thick atmosphere, which allows flight to be performed with less energy, and permits the existence of air-borne phytoplankton.

Much of the plot conflict stems from the love triangle between the protagonists and the mysterious motives of a cybernetically-augmented government agent sent along to monitor their progress.

A secondary story tells of a multidimensional cellular automaton energy being named 0X[1] and its attempts to share living space with an infant human male, a fledgling creature of Orn's species, and one of the manta-carnivores; their developing relationship leads to attempts to learn the reasons for their strange isolation from others of their kind, which eventually ties into the story of Cal, Veg, and Aquilon.

Lester del Rey found Orn to be "the most enjoyable book by Anthony" to date (1972), an improvement "in many ways" over Omnivore.