The book explores the aftermath of the crucifixion of Jesus through the experiences of the Roman tribune, Marcellus Gallio and his Greek slave Demetrius. Prince Gaius, in an effort to rid Rome of Marcellus, banishes Marcellus to the command of the Roman garrison at Minoa, a port city in southern Palestine. In Jerusalem during Passover, Marcellus ends up carrying out the crucifixion of Jesus but is troubled since he believes Jesus is innocent of any crime.

Marcellus and some other soldiers throw dice to see who will take Jesus' seamless robe. Marcellus wins and asks Demetrius to take care of the robe.

Following the crucifixion, Marcellus takes part in a banquet attended by Pontius Pilate. During the banquet, a drunken centurion insists that Marcellus wear Jesus' robe. Reluctantly wearing the garment, Marcellus apparently suffers a nervous breakdown and returns to Rome.

Sent to Athens to recuperate, Marcellus finally gives in to Demetrius' urging and touches the robe, and his mind is subsequently restored. Marcellus, now believing the robe has some sort of innate power, returns to Judea, follows the path Jesus took, and meets many people whose lives Jesus had affected. Based upon their experiences first Demetrius and then Marcellus becomes a follower of Jesus.

Marcellus then returns to Rome, where he must report his experiences to the emperor, Tiberius at Villa Jovis on Capri. Marcellus frees Demetrius, who escapes. However, later on, because of his uncompromising stance regarding his Christian faith, both Marcellus and his new wife Diana are executed by the new emperor, Caligula. Marcellus arranges that the robe be given to "The Big Fisherman" (Simon Peter).