Blood Brothers: Book One of the Sign of Seven Trilogy opens in 1652, in Hawkins Hollow, Maryland. Giles Dent and Ann Hawkins are accused of witchcraft by Lazarus Twisse, a demon hiding in the shell of a man. Twisse initially heard the accusation against the two from a young girl named Hester Deale. Giles sends Ann, who is pregnant with triplets, away before Lazarus arrives. Twisse’s followers descend on Giles with torches, but as Giles burns he pulls Twisse into the flames with him. 

More than 300 years later, in 1982, the area where Giles and Twisse burned is known as the Pagan Stone. Three best friends—Cal, Fox, and Gage—hike to the Pagan Stone the day before their shared tenth birthday on July 7th. On the way, they stop for a swim in Hester’s Pool, named for a girl who drowned in the pond. While underwater, Cal sees a figure that he somehow knows is Hester. At the Pagan Stone, the three boys decide to make a blood oath of loyalty to each other at midnight. That night, as they make the oath, they rise off the ground as wind and fire swirl around them.

Nearly twenty-one years later, Cal and Fox still live in Hawkins Hollow, while Gage travels around looking for unexplained events similar to what happened to himself and his friends as children. Every seven years since their tenth birthday, people in Hawkins Hollow are possessed by some sort of evil force and do things they otherwise would not, such as set fires, destroy property, and commit murder. For some reason, Cal, Fox, Gage, and their families are never hurt or affected during this time, and the three friends also never get sick or sustain injuries. Because the event occurs on the seventh day of the seventh month every seven years, the town has come to call it “the Seven.” Now, it’s the February before their thirty-first birthday, and Cal and Fox have been having bad dreams. They fear that whatever force takes over the town is coming earlier and will be stronger.

Cal meets with Quinn Black, a writer interested in learning and writing about Hawkins Hollow. Quinn tells Cal that as she drove toward town she saw a figure that she thought was a boy or a dog, which then disappeared. Cal wonders if Quinn has ties to Hawkins Hollow. That night in her hotel, Quinn dreams that she is at the Pagan Stone. Whe she wakes up, Quinn sees a red-eyed boy floating outside her window. The next day, Quinn visits the library to learn more about the town and talks to Estelle Abbott, the elderly library director. Estelle tells Quinn about what happened in 1652 with Giles Dent and Lazarus Twisse and says that Ann Hawkins eventually returned to Hawkins Hollow with her three sons after Twisse and Giles burned, and that Hester Deale had a daughter before drowning herself in Hester’s Pool.

While having dinner in the hotel restaurant, Quinn sees a slug-like creature crawl onto a nearby couple’s table. No one else sees the creature except for a young woman who has just entered the restaurant. She and Quinn run outside, and the woman introduces herself as Layla Darnell. Quinn brings Layla to see Cal and Fox, and Layla explains that even though she lives in New York, she kept having nightmares and heard the words “Hawkins Hollow.” So, Layla rented a car and found herself driving to the town. They wonder if Layla, like Quinn, is also somehow tied to Hawkins Hollow.

Cal takes Quinn to see the Pagan Stone, which Quinn recognizes from her dream. While standing by the stone, both Cal and Quinn are overtaken by a mysterious force, and they kiss. They agree that neither of them was in control when they kissed, but also agree that they like each other. While Quinn explores the area, Cal has a vision of Giles Dent and Ann Hawkins right before Giles sent Ann away. Cal sees that Giles was wearing a bloodstone amulet, which he recognizes as the same as the bloodstone that he, Fox, and Gage each have a piece of.

Quinn and Layla decide to rent a house since they do not know how long they plan to stay in Hawkins Hollow. On Valentine’s Day, Quinn, Layla, Cal, and Fox go to the Sweetheart Dance at the bowling alley that Cal’s family owns. While there they see blood and spiders coming out of the decorations, although no one else can see them. Afterward, Cal and Quinn have sex for the first time.

Cal’s great-grandmother gives Quinn journals from old family members, some of which belonged to Ann Hawkins. Quinn and Cal wonder if Cal, Fox, and Gage are related to Giles Dent and Ann Hawkins. Gage arrives at Cal’s house after traveling in the Czech Republic, and Quinn’s friend Cybil arrives to help Quinn investigate the town. While exploring a cemetery, Quinn sees the gravestone of Joseph Black and wonders if she could be related to him. Quinn looks into families of Hawkins Hollow and finds that there was a family named Clark, which Layla says is her grandmother’s maiden name. Meanwhile, Gage tells Cal and Fox that he has learned about a demon called Tmavy, or “The Dark,” who can take on different forms and cause people around it to go insane. They believe that this is the demon that has been terrorizing Hawkins Hollow, and they wonder if Giles Dent somehow gave them the power to destroy it.

When a blizzard is forecast, Cal invites everyone else to stay at his house. While there, Quinn receives an email from her grandmother that contains a photo of the names of her ancestors written in the family Bible. One of her ancestors is Sebastian Deale, Hester Deale’s father. One of Hester’s granddaughters married a man named Duncan Clark, who is Layla’s ancestor. Based on their matching visions and dreams, Quinn and Layla know that the demon raped and impregnated Hester, which is why she killed herself after giving birth. Quinn fears that she, Layla, and Cybil are capable of evil because they are descendants of the demon.

After the snow melts, the six friends head to the Pagan Stone. As they approach the stone, they see the demon in the form of a boy. Gage shoots at the demon while Cal yells at it, telling it they will not go away. Then Cal takes out his knife and stabs the demon, which looks at him angrily before disappearing. The six know they haven’t destroyed the demon completely, but they feel confident that they will be able to before the Seven arrives.