Immortal is the story of Luca Bastardo, or Luca the bastard, a man living in 14th-15th century Florence. Luca tells the story himself, writing his memoirs as he prepares to die. What he produces is a pleasant, if protracted, trip into the an important era in Western civilization. Filled with the art, philosophy, and political intrigue of the day, Immortal paints a rich Renaissance landscape.

Luca's reminiscences begin when he is nine years old and a homeless orphan scrambling to live on the streets. "I never knew where I came from. It was as if I woke up on the streets of Florence in 1330, a boy already grown nine years." Although he is a street urchin, people remark on his beauty and assure him he must have been well-bred. He hears tales of a noblewoman, with hair the same rare red as his, looking for a lost child. He also hears of a letter, written by a cleric, that will condemn him should it ever reach the hands of the Catholic church.

What follows are Luca's trials as he grows older more slowly than those around him, and his search for his family. He lives through several rounds of the bubonic plague and befriends prominent figures of the time, such as Leonardo Da Vinci and the Medeci family. His first artist friend is Giotto, who tells him something he remembers the rest of his life: "God laughs." For a long time, Luca takes this to be mean-spirited laughter, and he comes up with a theory of two Gods: one good and one evil.

Luca's idea about two Gods is not unique; it is a concept of a heretical Christian group called the Cathars, something to which author Traci Slatton refers many times. The Cathars believed that the God of the Old Testament was an evil God who created the physical world, and a good God created the human soul, which is trapped in the body. The goal of life for the Cathars was to liberate the soul from the body by living a pure life: no killing, no eating meat, no sex. Those who did not achieve purity in this life reincarnated until they did. The Catholic Church perceived the Cathar movement to be a serious threat, and Pope Innocent III proclaimed a crusade against them.