The Mark of the Thief, published in 2015, is the first book of Jennifer A. Nielsen’s middle grade/young adult Mark of the Thief series. The first installment introduces readers to Nicolas “Nic” Calva, a boy from Gaul who became enslaved in the mines outside of Rome after the conquest of his people. When Nic finds the coveted bulla of the deified Caesar, he absorbs its magic and becomes the target of two opposing sides, each of which will do whatever it takes to possess the treasure and its power. As Nic becomes caught in the crossfire, he must decide what he stands for and who he wants to stand beside.

Nielsen is a New York Times best-selling author of seven middle grade/young adult historical fiction novels and three middle grade/young adult fantasy series, among others. She is the winner of numerous awards, including the Sydney Taylor Notable Book Award, an Outstanding Achievement Award, and multiple Whitney and state book awards.

This guide refers to the 2015 Scholastic Kindle e-book edition. 

Content Warning: The novel contains themes of enslavement. This guide uses “slave” only in quoted material to reflect its use in the text.

Plot Summary

Nic, the “thief” referenced in the book’s title, narrates his journey in Rome from the mines to the highest echelons of power as he strives to master his magic and fate.

The novel opens with Nic’s enslaver, Sal, ordering him to explore a cave believed to hold Julius Caesar’s lost treasure. Nic objects on the grounds that lives and the ability to think rationally have been lost in the search for it, so he hides in the tunnels. There, he overhears prominent Roman General Radulf—who is from Gaul, like Nic—plotting to destroy the empire. Nic is found and forced into the cave, where he finds a glowing bulla (a pendant containing an amulet worn by freeborn boys in Ancient Rome) that belonged to Caesar. When he grabs it, a griffin attacks and wounds him, but when he holds the bulla, the griffin stops fighting him, and he names her Caela. When the cave begins collapsing, Caela flies them out to safety. Later, the guards find them and accuse Nic of trying to escape. A high-ranking senator named Valerius and his son Crispus arrive and attempt to buy Nic, who has the bulla hidden under his tunic; however, the guards insist on returning Nic and Caela, who they have tied up, to the mines. Before Nic leaves, Valerius sees a mark on his back where Caela scratched him and warns him not to trust anyone in Rome.

Sal is furious upon Nic’s return and intends to kill him, but Nic’s sister Livia, who Sal desires, agrees to marry him if he lets her brother live. Sal accepts her offer but then gives Nic and Caela to Felix, who works for the brutal venatio (public games that feature contests between beasts or men and beasts) in Rome. After inspecting Nic, Felix notices the mark on his back and warns Aurelia, a girl he assigns to guard Nic, to keep her distance. Nic notices her crepundia (a series of charms given to children) and realizes she must be a plebeian, or commoner. During their journey, Nic discovers that he possesses unnatural strength and the ability to communicate with Caela.

When they arrive in Rome, Felix warns Nic that the mark will mean trouble for him. The beauty and majesty of the architecture awe Nic; he feels like he is part of this city, since it was built from the materials he has mined. Felix points out the sites and tells him Rome was founded in bloodshed and betrayal. When they arrive at the amphitheater, Nic learns that Caela will be hunted to her death in the games. In betraying the griffin, he feels he has truly become part of Rome. Later, Felix tells Nic that Caesar’s claim to be descended from Venus is true, and the mark Nic bears is the emperor’s Divine Star, which carries the power of the gods. The bulla holds power as well, and Felix warns Nic not to show it to anyone, advising him to give it to the emperor, Tacitus.

Crispus visits and asks to see Nic’s magic. When Nic argues that he knows no magic, Crispus responds that he must learn how to use it, and Valerius can help and protect Nic. Aurelia helps Nic understand the danger he is in, since he can neither give up nor keep the bulla without dangerous consequences. When the games are underway, Felix has Nic sent in to be killed so that his bulla will be given to Tacitus. Nic uses his ability to communicate with animals and his superhuman strength to escape with Caela, but both of them are injured in the process and they become separated. Nic takes shelter in the sewers under Rome, where he discovers Aurelia. His wound has become badly infected. Aurelia treats it, saving his life, but ultimately, she decides to turn him over to a Roman senator named Horatio in exchange for reward money. After Aurelia shackles Nic, Radulf finds their hideout and demands Nic turn over the bulla. Nic is able to use his magic to escape with Aurelia and the children, who are sent to another safe haven.

Back above ground, Aurelia and Nic seek out Felix to unlock the manacles Aurelia had Nic bound with, since they originally belonged to him. When Felix learns Radulf has magic, he unlocks the manacles, revealing that Radulf, like Nic, was enslaved after the conquest of Gaul and rose to popularity and prominence as a gladiator. Nic agrees to give the emperor the bulla after he exposes Radulf. Later, guards spot Nic and Aurelia and corner them in a temple of Julius Caesar, but Crispus sneaks them out in disguise and brings them to his father’s house. There, Crispus and Aurelia help Nic practice his magic.

Valerius tells Nic about two other amulets that hold divine power—one of which is held by Horatio—and that Radulf intends to acquire all three to destroy Rome. Radulf controls the military and the Praetorian Guard, who are loyal to Horatio, not Tacitus. Nic must gain control of his magic to save the empire. Nic is given fine clothes, sandals, and his own room with a bed, all of which change his self-image. Aurelia helps him understand that he must believe he is equal to Radulf to fight him effectively.

Nic uses his magic to heal himself and discovers that it is tied to his emotions. The key to controlling one is controlling the other. Later, Valerius reveals that Horatio intends to announce his loyalty publicly at the next games and insists they must stop him and Radulf whatever the price, which troubles Nic. In the meantime, he learns where Caela is and finds her wounded and dying. He is able to heal her, but Radulf lures Nic into the open and sets fire to the surrounding houses, making it look like Nic was responsible for the damage. He taunts Nic, claiming that he will always be stronger because he has no loyalty except to power.

The damage his fight with Radulf caused upsets Nic, and he becomes angry when Valerius cares only that he held his own against the general. At the games, Valerius plans to pressure Horatio into supporting the empire by holding up his family banner and affirming Horatio’s support of the emperor. Nic doubts the plan will work but is more troubled to notice that the symbols on the banner match Aurelia’s crepundia. He confronts her for hiding that she is Horatio’s daughter. Just then, Horatio arrives to arrest Nic—who he found through Sal, not Aurelia. Horatio brings Nic back to his house and admits that he wants Rome destroyed because it could not save his wife: She died giving birth to Aurelia, after which Horatio exposed the baby. He does not lock Nic’s prison room and is surprised when he chooses not to escape. Nic is determined to face Radulf in the games the following day.

Before entering the arena to fight Radulf, Nic learns from Felix that Valerius intends for Horatio to die. The arena is designed to give Nic no escape but to fight directly with Radulf. When they face off, Radulf initially gains the advantage, but Valerius draws his attention away by claiming that Horatio has given one of the other two amulets to Nic. It is not true, but the ruse works as Radulf turns his attention to Horatio, who he ultimately kills, leaving Nic time to protect himself and eventually escape with Caela. He goes to the emperor’s palace to give the bulla to Tacitus, but he discovers Radulf and Livia there instead. Tacitus has gone on campaign, where he will be killed. Radulf is revealed to be Nic and Livia’s grandfather, and he compels Nic to join him by threatening to kill Livia. Radulf takes possession of the bulla but does not realize that Nic has removed the magical gems that power it.