In the novel, Satan is offered a human body for one month, provided that he does nothing which will permanently harm the host. If he can live out the month blamelessly, he can keep the body until its eventual demise. If he lives virtuously and repents, he can return to Heaven, regaining his angelic status.
His host, Declan Gunn (an anagram of Glen Duncan), is a depressed writer who is preparing to commit suicide. Through the aid of his minions, Lucifer procures a vast amount of money and proceeds to live a rock-and-roll life style, immediately selling the film rights to the Fall of the Angels and his war with Heaven.
The premise of the novel is that Lucifer is writing it on Gunn's computer at the same time as the screenplay. He frequently flashes back to events in both his and Gunn's lives, ranging from the temptation of Eve to the release of Gunn's last novel. As the novel progresses, Gunn's life interferes more and more with Lucifer's, while Lucifer seemingly becomes more human.
The novel itself digresses repeatedly, examining such topics as hypocrisy in the Spanish Inquisition, the humanity in figures that we have come to label as inhuman (using Heinrich Himmler as an example), the circumstances leading to Satan's fall from grace, and how Elton John is a possible descendant of the Nephilim.