Honoré is walking down the street, preparing to meet with Emily and pondering how he will make a living once rationing comes to an end, when an elderly medieval knight suddenly materialises out of thin air, attacks him, and passes out. Honoré reads the man's timeline and discovers that he has travelled through time from the 11th century. When Emily arrives and Honoré tells her what's happened, she becomes intrigued and convinces him to investigate. Together, they leap back along the man's time-snake to the camp of Godfrey de Bouillion, one of the crusaders laying siege to the city of Antioch. The newcomers' strange clothing attracts attention, and Honoré's skin colour marks him out as a Saracen -- and when a man is found dead in the camp with a look of terror on his face, Honoré and Emily are accused of murder.
Godfrey has Honoré thrown into a pit to await punishment, but a kind young knight named Simon offers to place Emily under his protection, apparently hoping to convince her to reject the "devil-worshipping" Saracen with whom she's been associating. Emily eventually manages to convince Godfrey that she is not responsible for the death, and he allows her to examine the body of this man and the other, similar victims that have been found. The bodies have strange puncture marks on their necks and have already begun to decompose, as if the energy binding the bodies together has been drained out of them. Simon introduces Emily to Alice, a woman who claims to have had a religious vision; however, while Alice is convinced that she saw St Luke arrive in the camp, Emily privately believes that Alice in fact witnessed the time-traveller's arrival.
Emily convinces Simon to release Honoré, and together, they question Bishop Adhemar of Le Puy, who claims to have seen the dead walking between the camp and the city. Their investigation stalls at this point, but while they are discussing their next move, Honoré and Emily hear a scream and find themselves frozen in place, unable to move as a strange slurping sound echoes from outside. Honoré forces himself to move, and catches a glimpse of a large, slug-like creature slithering away from its latest victim. Pinned beneath the body is a coin from Antioch, and Honoré thus asks Simon for permission to lead a small raiding party into the city, partly for supplies and partly to investigate this lead. Simon insists that Emily remain in the camp, where it's safe, and Honoré shakes his hand to seal the deal -- but as he does, he catches a glimpse of Simon's time-snake, which is unlike any other he's ever seen. When he takes a look at the time-snakes of the nearby knights, he sees that they've all been unnaturally truncated, as if something is about to cause them to die before their time.
Honoré sneaks into the city, accompanied by a preacher named Peter and two of Simon's vassals, Timothy and Edward. Seeking to avoid bloodshed, he convinces the reluctant Christians to pretend to be his servants, and they remain silent while he strikes a deal with one of the Saracen commanders, Firuz, to trade food and supplies with the crusaders in exchange for an agreement to negotiate terms. At the last moment, however, Timothy and Edward turn on their escort, kill them and open the gates of the city. Edward is killed in the subsequent fighting, but the crusaders successfully breach the gates and break the siege.
Furious with himself for his gullibility, but unable to prevent the ensuing slaughter, Honoré sets off with Peter to investigate rumours of a Christian knight living at the Church of St Peter in the heart of the city. The knight in question turns out to be a younger version of the elderly knight from 1950, and Peter identifies him as Reynald, former Earl of Marseille and traitor to the Christian cause. Timothy arrives, recognises Reynald and tries to kill him, but is halted in his tracks by the manifestation of a skeletal knight. Reynald banishes the spectre with a spearhead that the horrified Timothy concludes must be the Lance used to pierce Christ's side -- and while Timothy is distracted, Reynald stabs him with the Lance, draining the life out of his body and leaving only a rapidly decaying corpse. He then attacks Honoré and stabs him in the leg with the Lance, but their close proximity allows Honoré to read Reynald's time-snake -- while, at the same time, Reynald is able to read Honoré's. Not only are they both time-channellers, but they share a common ancestor.
Emily leaves the Christian camp to look for Honoré, and leads a group of crusaders to the church, where they overpower Reynald just in time to save Honoré's life. The tide of the battle then turns as Saracen reinforcements arrive from Kerbogha, but despite the danger, Honoré insists that Emily find Simon and try to convince him that they have a more important battle to fight, in the church. Emily makes her way back to the crusaders' camp, and as she tells Simon her story, a slug-like creature materialises out of thin air and attacks Simon's vassals, killing one and injuring another. Simon asks Emily to stay and tend to the injured Aimery while he enters the city to deal with Reynald, but Aimery dies despite her efforts, and she thus follows Simon into the city.
Honoré returns to the church to find that Reynald has killed his captors and is using the Lance to summon the true object of his worship: the Fendahl. The summoning causes a localised time-storm that throws Honoré out of the church, and he awakens to find Fendahleen roaming the streets, sucking the life out of anyone they encounter. Simon contacts Honoré and claims that only Honoré can defeat Reynald, as the other knights are too frightened to confront the renegade -- but when they return to the church, they find it littered with bodies, and Honoré realises that Simon lied to him. Though Simon is clearly no coward, he seems determined not to confront Reynald himself; convinced that Simon is hiding something, Honoré forces the issue by overpowering Simon and flinging him at Reynald. Reynald stabs Simon with the Lance, but instead of dying, Simon transforms into an angelic being, similar to the time creature Honoré and Emily encountered in 1921. Simon was sent back in Time to stop the Fendahl from manifesting here, but thanks to Honoré's interference, Reynald has accelerated his plans and is now close to success.
Reynald takes the Lance to the roof of the church, where he is struck by lightning and completes his transformation into the Fendahl Core. Emily arrives and attacks him, but he tries to convince her to join him and spread the Fendahl's reign of death throughout all time and space. Horrified to realise that part of her finds this appealing, Emily prepares to sacrifice herself by jumping through time with Reynald to take him away from this place. However, upon realising what she intends to do, Simon finally takes responsibility for his own fight, moves Emily to safety and attacks Reynald himself. As Simon tries to force Reynald through Time, Reynald struggles to resist, causing his own personal timeline to advance; as Emily and Honoré watch, Reynald ages decades within seconds, and merges with Simon's own timeline as he does so. Finally, the two beings become one, and Reynald becomes too old and weak to unleash the full force of the Fendahl within him. Reynald and Honoré share one last glimpse of each other's timelines, and then Reynald disappears; presumably Simon has taken him into the future to meet Honoré in 1950 and ensure that these events take place.
Without Reynald, the Lance is a harmless shard of bone; Honoré gives it to Peter, who intends to use it as a symbol to inspire the crusaders to victory. Honoré and Emily then make a series of leaps forward along people's timelines until they're back home in 1950. The elderly knight has vanished, and Honoré and Emily are unsure whether Simon has taken Reynald to a place of safety or merely shuffled the problem of the Fendahl off onto someone else. In any case, they now have other things to fear; Honoré saw nothing in his future when he looked through Reynald's eyes, and Emily is concerned that she found the concept of mass death appealing in some way. Perhaps she has amnesia because, like Simon, she's trying to reject her true nature -- and perhaps she's right to do so...