In the series, vampires, witches, werewolves, and shape-shifters live among humans without their knowledge, making up a secret society known as the Night World.
Each volume in the series follows a different teenage, female protagonist who must face challenges involving love, the "soulmate principle", and the Night World's strict code.
Literature scholar Deborah Wilson Overstreet describes the so-called "soulmate principle" as "the main story line in each novel".
James hides Poppy in his apartment, but his "crazy" cousin Ash Redfern abducts her, intending to expose her as an illegal vampire at a gathering in Las Vegas.
Resolving to change their people's laws, James and Poppy leave to seek help from the latter's father, the source of the Norths' witch ancestry.
Lamia sisters Rowan, Kestrel and Jade Redfern have fled their homeland, a Night World island enclave, for Oregon.
Opal's neighbor Mary-Lynnette Carter suspects the sisters of murder, but after she and her brother Mark find them eating a deer in the woods, they reveal their vampire heritage and convince her they are innocent.
Instead of following Night World law by killing the Carters, the Redferns do a "blood-crossing" ceremony, binding the families in a vow of mutual protection - to the delight of Jade and Mark, who are convinced they are soulmates.
Meanwhile, the Redferns' ruthless brother Ash has followed them to Oregon to either retrieve or kill them; however, when he meets Mary-Lynnette, the two realize to their joint horror that they are also soulmates.
However, Jeremy ambushes them in werewolf form, and reveals that he is the killer; he resented Opal for bringing three new vampires into his hunting territory, and when the Redferns wouldn't leave, he killed the animals to scare them off.
Mary-Lynnette admits she loves him but is hesitant to become a vampire, and Ash vows to reflect on his wrongdoings to become worthy of her; they agree he will return to see her next year.
He encourages her to alter her appearance, her behavior and her social circle, making her the most popular girl at school and uniting her with her long-time crush, David Blackburn.
Angel also reveals Gillian's heritage; she is descended from one of two lost babies from the Harman family, making her a witch, and with this insight he helps her create a spell to stop Tanya and her accomplice Kim from carrying out their plan.
He guides her to a hidden Night World store to obtain supplies, earning disapproval from its witch storekeeper Melusine, who realizes Gillian will be using dark magic.
Angel's commands become stranger, more people end up hurt, and eventually he guides Gillian to an unsavory Night World clubhouse where she is almost mauled by werewolves; she is saved by Ash Redfern.
Together they undo the spell against Tanya and Kim, and conclude that Angel is likely a spirit with unfinished business who died close to where Gillian met him.
She tries to find Angel in the local cemetery, and eventually he appears and takes her to his grave; his real name is Gary Fargeon, and he is descended from the other lost Harman baby, making him Gillian's second cousin.
He has remained a spirit because he accidentally killed a young girl while casting an unstable spell, and died in a drunk-driving crash without telling anyone what he did.
Aged just five, Rashel Jordan witnesses the murder of her mother and her friend Timmy by a ruthless vampire, then survives a fire that kills her one remaining relative.
Rashel stays behind to find out who ordered the abductions but is attacked by one of the werewolves; she is saved by Quinn, who has concluded that their connection means they are soulmates, and attempts to turn her into a vampire to avoid breaking the law against loving a human.
Rashel instantly recognises him as her mother's killer, and learns that Timmy is not dead, but was turned into a vampire and raised by Hunter as a tormented eternal four-year-old.
Rashel's rage towards vampires subsides, and she and Quinn decide to search for a rumored coven that accepts humans, Circle Daybreak.
Circle of Daybreak shapeshifter Raksha Keller and her team race to a mall to save the new Wild Power, Iliana Harman.
Iliana's deaf friend, Jaime, is hosting a party, to be held on the same night as the ceremony that bonds the witches and shapeshifters.
Galen who has never acquired a shape yet and wanted to be something small and peaceful turns into a leopard and removes another horn to save Raksha.