Marius is a young Roman whose family has for many years lived on Whitenights, an estate in northern Italy. On that estate, Marius had grown to adolescence in an atmosphere of pagan piety and rural simplicity. The family leads a relatively simple life because Marius’s grandfather had squandered much of the family fortune. In the atmosphere of his childhood, Marius finds a great joy in worshiping the household gods and in overseeing the work on the estate. His life is one of contemplation rather than one of activity, and his idealism and religiosity are almost morbid in their extreme.

While still in his teens, young Marius is taken to a temple of Aesculapius in the Etrurian hills for the cure of a childhood disease. There the quiet, fresh atmosphere of the place, as well as the teachings of Galen, the great Roman physician, give him a new outlook on life. Upon his return home, Marius finds his mother’s health failing. She dies shortly afterward, and the effect of her death on Marius is to turn him into a skeptic, a young man who questions all aspects of life as they present themselves to him.

Soon afterward, relatives send young Marius to Pisa, where he attends school. While there, he conceives the idea of becoming a poet of the intellectual school. His inclination in that direction is stimulated by his friendship with Flavian, a schoolmate. Flavian is three years older than Marius and has great influence over the younger boy. The two read all the literature and philosophy they find. Among the works they pore over is the Metamorphoses of Apuleius; its ornate style is a source of great joy to Marius.

The studies in literature and philosophy that the two young men plan, however, are short-lived. Flavian gets sick after an excursion with Marius, and he dies soon after of a plague brought back to Italy by the armies of Marcus Aurelius, who had just returned from a campaign into the eastern reaches of the Roman Empire. After Flavian’s death, Marius needs an intellectual crutch to carry him through the agony of seeing his young friend die, and he becomes attracted to the study of mysticism. At last, he puts aside the desire to look to Asian mystic lore and turns to early Greek philosophers to find some answer to his problems in their writings and thought.

One of the first writers he studies is Heraclitus, who teaches him to limit his labors so that he will not lose everything by trying to master all knowledge at once. From Heraclitus, he turns to the teachings of Aristippus of Cyrene, founder of the Epicurean school. From his study of Cyrenaic philosophy, Marius concludes that knowledge is limited to experiences received through the senses, and he thinks that he owes it to himself to have many sensuous experiences to reach the highest possible point of wisdom.

The idea appeals to Marius because of the immensely practical ethics that the whole concept implies. Life as the primary end of life is the code that Marius finds himself professing; it is, of course, an antimetaphysical metaphysic. Through it, Marius hopes to find, by means of cultural knowledge, the secret of the present in the changing universe; he wants to discover all the subtle realizations implied in each moment of life. Like Epicureans of that time and since, Marius finds there are those who misinterpret his credo and believe that he seeks pleasure as an end in itself; yet hedonism, the search for pleasure as the purpose of life, is farthest from his mind. Such a life would have been too gross for one of Marius’s pietistic background. During his search for an answer to life, Marius turns from poetry to prose, which he feels better fits his nature and his studies.

About the time that his Epicureanism became crystallized in his mind, Marius feels some pangs of regret that his emotional life seems to have become stunted. He wonders why it is that he feels more inclined to research of the mind than to normal human emotions. He cannot feel the necessity of pursuing feminine company and does not regret that he has not found it a matter of urgency that he acquire a wife. Love, in the ordinary sense of the word, does not seem to be a part of his makeup.

At a time when this problem is disturbing him, he has a summons to Rome that interrupts his worries. He is called to become secretary and editor to Emperor Marcus Aurelius, a prolific writer and a patron of the arts and philosophy. Marcus Aurelius has been working for some time on a memoir and a series of disconnected meditations that he wished someone to put into edited form. That task was assigned to Marius.

On the way to Rome, Marius meets a young officer of the army named Cornelius, an officer of the famed Twelfth Legion, who is returning to Rome after service in the farther reaches of northern Europe. Under the tutelage of Cornelius, Marius quickly makes himself at home in the city. Fortunately, Marius’s family has a house in Rome, although it has not been used in many years. To the young Epicurean, Rome is a wonderful place in which to live; for several years, Marius is happy there. Experiences of the richest nature are his; thanks to his family background and the emperor’s patronage, he moves in the best of circles.

There is, however, something that Marius cannot fathom. His friend, Cornelius, seems much happier than he. Since Cornelius is not a simple materialistic person, Marius cannot understand why his friend is so much happier. One day as they are returning from a trip away from Rome, Cornelius takes Marius into a rich home on the Appian Way. It is the residence of the widow of Cecilius. Cecilia, the home’s mistress, is a Christian, as is Cornelius. From that moment, Marius begins to comprehend something of the new religion that is making converts in the empire. He finds a strange kind of happiness in attending mass in the home of Cecilia, and he also feels a strange attraction to Cecilia herself.

Some months later, when Cornelius and Marius are once again away from Rome, the small town in which they have stopped was shaken by an earthquake. After the first tremors of the quake pass, Cornelius, accompanied by Marius, joins a group of Christians who are publicly thanking the Deity for their escape from death. Fearing that the Christians caused the earthquake, the pagans of the town assault them. Marius and Cornelius are arrested because of their rank and sent to Rome. On the way, their captors learn that one of them is not a Christian. To save his friend, Marius says that the non-Christian is Cornelius, who is then set free. Marius himself becomes violently ill before he and his guards reach Rome. He is left behind to die, but some villagers, who are also Christians, find him and nurse him. He dies with Christian prayers in his ears.