After being attacked by a demon as she leaves work, Cassidy Long is saved by an enigmatic man and spirited away to safety. However, after awakening in unfamiliar surroundings, she fears that instead of being rescued she has been kidnapped.

Finding herself alone, Cassidy takes the chance to escape her perceived captors. The attempt is short lived when Rail finds her. Rail is chastised by his friend, Boomer, for allowing Cassidy to escape, and the two men break into a fight; a fight which reveals their true natures. Cassidy recoils in fear as both men transform into demons.

As Rail acknowledges his over protectiveness for Cassidy, he must also acknowledge the fact that she is the current vessel of his soul—a fact that both elates and frightens him. Until now every one of his soul-keepers has been a faceless, nameless stranger, and now he has a personal connection to one. He’ll do his duty to protect her but worries that he won’t be able to fight the emotional storm he feels growing between them.

It takes a lot of explaining on Rail’s part to get Cassidy to understand that he is not insane when he tells her that he and his companions are the last line of defense against Lucifer’s demons. He admits that while they are technically soulless Fallen Angels, they are not evil.

In ancient times they were pure angels, but when they objected to the utter destruction of their human loved ones and children they were punished. At the time of their exile, their souls were ripped out and cast into the unknown, falling into random unborn humans. Rail then reveals that Cassidy is the current keeper of his soul.

At first, Cassidy balks at being a vessel for Rail’s soul let alone believing his story. Yet she can’t dispute that she is attracted to him. While discussing what it means to be his souls-keeper, Rail inexplicably collapses in pain. Fearing that Rail is being punished by a higher power, Cassidy acknowledges that she can feel the soul-connection and vows to help him.

During Rails recovery, Cassidy surmises that the Knights are under a compulsion that prevents them from thinking about searching for their soul-keepers. As she explains her hypothesis Rail, and his two companions Boomer and Kaz, stare blankly into space, recovering only when she stops talking.

Putting a stop to the discussion, Kaz insists it’s time to leave their hiding place and head for home. Cassidy refuses to go willingly unless they let her contact her parents to say good-bye. Kaz refuses, but Rail soon convinces him that Cassidy will be even more trouble if she doesn’t get her way in this. After a tearful phone call to her parents, Cassidy agrees to go into hiding from the demons that hunt the soul-keepers.

Cassidy isn’t alone on the island. There are already several other soul-keepers under protective custody living there. Any thoughts of trying to leave are quashed when she stumbles upon the injured and suicidal Margarite, another soul-keeper and recent victim of a demon attack. It’s a heartbreaking realization to Cassidy how close she came to being killed herself.

As Rail tries to convince Kaz that Cassidy should be allowed to use her researching skills to find others of their kind they are interrupted by a terrifying scream. Cassidy has stumbled upon the childlike demon servants that the Knights keep hidden from the humans living on the island. After her initial shock wears off Cassidy is once again at odds with Kaz, insisting that everyone know about the servants and that the little demons be allowed to live in the open.

Despite the upheaval that Cassidy’s changes have made on the island Rail leaves on a mission to Peru to investigate a possible demon attack and is killed in battle. When he returns to life three days later, Rail is consumed with the need to see Cassidy. Returning to the island, he finds her on the beach, and they finally give in to their passions. As they make love for the first time, their souls bond on a level only whispered of in legends.

To help Cassidy in her research project Rail and several of the other Knights break into Cassidy’s former workplace to retrieve some files she believes will assist her. To their surprise, one of Lucifer’s Minions is waiting for them. During their attempt to escape Cassidy is killed. To everyone’s astonishment, Cassidy is resurrected, much in the fashion of the Knights, within a day.

With Cassidy’s return to life, Kaz reveals snippets of a prophecy which leads the Knights to believe the soul-mating is the first step to the end of the war between Heaven and Hell. The attack on Cassidy also shows that Lucifer is also aware of the prophecy. The question is how had he found out about Cassidy?

Tension runs high on the island as implications of a traitor become evident. One of the demon servants confesses that she passed on information that leads to the attack. Rail is consumed with rage and sentences the demon to death at the hands of her fellow servants. Cassidy is appalled by the display of violence. In her anger and confusion, Cassidy begins to question her relationship with Rail.

Giving Cassidy her space, Rail leaves on a mission. While in Alaska Rail experiences such pain that he fears an attack until he realizes all he is feeling is coming from Cassidy. Instinctively Rail follows their soul connection and finds Cassidy with her family. Somehow she has been transported from the island hideaway to her family home in Nevada.

During her impromptu visit with her family, Cassidy reveals the truth about the attack and why she cannot stay. During the discussion, several demons attack the house. After a brief battle, Cassidy and Rail have a heartfelt conversation as to their feelings. Rail reveals that although he loves Cassidy his fear of failing her is holding him back from giving in fully to the relationship.

Returning to the island Rail confronts Kaz with a need to change how they run things. He pushes Kaz to allow the humans to do more and assist Cassidy in her research. Rail also pushes for Kaz to open up the secret archives to everyone. Kaz sees this as a play for leadership and pushes back.

As she listens to the fight, Cassidy experiences the same sensations that she felt the moment before she transported to her parents’ home. Just as she vanishes, Rail grabs Cassidy and vanishes with her.

The horror of the situation is revealed when both Cassidy and Rail awaken in Hell. Lucifer himself has summoned them. He believes they are the key to his plans to return to Heaven. Not willing to bow to the Devil, even after Cassidy is gravely injured, Rail throws himself into battle with Lucifer. They are evenly matched, and the fight rages for hours.

When Rail succumbs to his injuries, Cassidy taps into their connection to bring him back to life. With their souls in complete harmony, the couple unwittingly merges into a creature that is both demon and angel. Fearing for his own existence, Lucifer cast them from Hell.

Back in the world of man, Cassidy and Rail are shocked to discover that at the moment they escaped Hell, so did dozens of demons. For twenty-four hours straight, the Knights have been moving from one battle to the next, helped by the programs that Cassidy has developed. During this time they have also discover evidence that there are other Fallen Angels out in the world still fighting.

To their great relief, the breach between worlds closes, and no further attacks seem imminent, but the cleanup will take months. During the respite, Rail returns to the secret archives to search for information on the bond he and Cassidy share. To his astonishment, he finds the full prophecy, written as a poem, hidden in a book accounting angelic visitations.

As a group, both Knights and their human companions set out to decipher what the prophecy means to them. Is it the key to ending the war, or ending the world?

Feeling that the worse is behind them, Rail confesses his love to Cassidy, requesting that they marry, to make their bond official. Of course, she says yes. There may be doubts as to the future, but the one thing they do know is that they will face it together.