In autumn of 1756, Lord John is dining at The Society for the Appreciation of the English Beefsteak with Harry Quarry. Harry teasingly toasts him about his return from exile – Ardsmuir Prison – a post they have in common. John's eyes keep wanting to look to the corridor, seeing a flash of red hair, a color that reminds him of someone he knew it couldn't be.
Quarry introduces the man as his cousin by marriage, Robert Gerald. Gerald is invited to join them for drinks while Quarry asks what had taken place in the corridor. Gerald explains that he had been made an offer from George Bubb-Dodington, an offer he refused. Upon leaving, Grey finds himself alone with Gerald in the vestibule. Gerald requests a favor of Grey – to meet after dark at the Royal Exchange, and Grey agrees, though he does not yet know the nature of Gerald's difficulty.
Grey fills his afternoon with errands, and Quarry joins him for company. The pair find themselves near the Exchange taking coffee, comparing London and how busy it is to their shared exile in the Highlands. The conversation turns to Fraser; Quarry speaks highly of the man, yet admits that Fraser wasn't a man to underestimate. He confides in Grey about the 'accidental' death of a Sergeant, which Quarry has documented as a 'misadventure' because, even if he could prove it was Fraser, he liked the Scot more than the sergeant.
The conversation between the men is interrupted as Quarry makes note of his cousin arriving. As they move to greet him, someone – using the surge of the crowd – strikes Gerald down with their blade. By the time that Quarry and Grey manage to push through the throngs of people, Gerald lay dying upon the ground. While Quarry rages at the crowd demanding answers, Grey tries to get even a hint of the perpetrator's identity from Gerald. Gerald's lips move to speak, but the words do not come as he dies. Grey vows to avenge Gerald.
Over the next two days, Grey makes inquiries and speculations, thoroughly inspecting every corner of Front Street, but to no avail. The most he can determine is that the attack was planned. Grey and Quarry make attempts to speak with the hall porter at the Beefsteak to determine what Bubb-Dodington had spoken to Gerald about the day of his death. However, the man was temporary staff and already gone. Grey tries canvassing all of Gerald's acquaintances for any rumors of enemies or falling outs that might reveal some hint of who would want to kill him, but no one can think of any reason someone would wish Gerald dead.
Grey receives an invitation to a party hosted by Sir Richard Joffrey and Lady Lucinda Joffrey, from Quarry, who has attached his own note to Grey to come, as he had news. Grey also receives a note from Bubb-Dodington requesting some of his time – as convenient – at the Joffreys'. Upon review of Quarry's note, he finds a broadsheet attached containing a rather invidious poem that gives undertones of speculation to Gerald's sexuality, and justice being served with his death.
Almost as soon as Grey arrives, Quarry takes him aside to speak in private. Quarry tells him how more of that broadsheet and others like it had been put all around the city, and that people were making accusations that Gerald was a pederast and a member of a notorious sodomitical society. These rumors only fuel Quarry's outrage.
Grey deduces that the murderer sought to discredit Gerald's name, as well as silence him – but why ruin the reputation of a dead man? Quarry, at a loss, looks around at the other guests; Grey does likewise, and is caught off guard by the sight of his former lover, George Everett. But Everett is a momentary distraction, and Grey is introduced to Lady Lucinda, who is doing her best to keep up appearances, in spite of her distress over her cousin's murder.
Feeling no longer obligated to keep silent about Gerald's last request, Grey tells Quarry and Lucinda that Gerald felt he could not confide in anyone else, and asked Grey to meet him. Quarry is shocked at the meeting's intended location, and offended that his cousin didn't confide in him.
Grey reminds Quarry that the original pretext for their meeting at the party was some news Quarry had. Quarry explains that he had learned that the offer Gerald had refused was an invitation to an event at Sir Francis Dashwood's home in West Wycombe. Lady Lucinda advises Grey, who is unfamiliar with the place, that Sir Francis entertains there for the purpose of seducing the powerful, much as many of the upper crust do. They consider that "the West Wycombe assemblage" might have been interested in Gerald's access to the prime minister.
While Quarry is drawn to the dance floor, Lucinda reveals to Grey that Quarry was exiled to Scotland after calling out a colonel for cheating at cards. She points out Sir Francis Dashwood across the room, and Grey recalls hearing about Dashwood's association, the Hellfire Club, that meets at Medmenham Abbey. After Lady Lucinda departs to join her husband, Grey moves through the crowd listening to the conversations, all about Gerald's death and the rumors about him. While recoiling from Justice Margrave's comments of castration as a punishment for sodomy, he is greeted by Everett. Their conversation is cut short when Bubb-Dodington makes his introduction to Grey, and invites him to come to Medmenham Abbey. Grey makes his excuses to leave the conversation and rejoins Lady Lucinda and Harry Quarry, who is still ranting in outrage about Dashwood. Grey notices the way Quarry's lips move to say 'Dashwood' and realized that's what Gerald was trying to say when he died.
With Dashwood's name now implicated, Grey accepts the invitation to Medmenham Abbey. For the first three days, Grey notices that there is an air secrecy, and other than women coming to dine, the house-guests are all men. The most Grey can determine for sure is that Dashwood didn't murder Gerald – at least not directly – as he has an alibi. Grey inquires bluntly of Everett both what Dashwood would want with him and about Gerald's death. Everett makes the point that by association, Grey was doing harm to his own reputation.
After a day of nervous anticipation, Grey retires as usual, passing the candle before the window to the signal to Quarry – who had insisted on accompanying him, but to remain at a distance – that all is well. Grey feels obscurely comforted by Quarry being outside, more so than by Everett's presence in the next room.
He is awakened to find his bed surrounded by what look like hooded monks, who pull Grey out of bed in silence, strip him, and help him into a robe of his own. He is led out to the garden, then through a curious portal carved to resemble the female privates. The 'monks' begin their chants, and Grey tries not to laugh at it all. Dashwood speaks, giving rites similar to mass but to invoke the Master of Darkness, and an ape dressed as a bishop is brought out, jumping onto the altar and slobbering over everything.
As the now drunken group makes to depart, Grey is seized and pulled back, and bent over a marble basin. Dashwood intones a prayer in reverse Latin, putting something warm and sticky over Grey's head. Grey's attempts to struggle result in his getting punched hard in the stomach, knocking the breath out of him. Now blood-stained, he is forced to drink wine with a distinct taste of opium in it, then taken to a room and pushed inside the door, which is locked behind him.
As the little opium he had ingested wears off, he is shocked to find a young, naked and quite dead woman in the room with him. Grey is trying to puzzle out the point of this, when the door finally opens and Everett enters the room. Grey demands to know what this is all about. Everett takes the rope from Grey's robe, leaving him exposed, and wraps it tight around the girls neck to make it appear that she had been strangled. Everett explains that it's part of the initiation to the brotherhood – the rites, the baptizing in blood, and then the new pledge is locked with a young woman for his pleasure before she is then to be sacrificed at the pledge's hands while an elder brother acts as witness.
Everett confesses that he had gone through the rites as well, and ultimately confesses to the murder of Robert Gerald; Everett had approached Gerald as a prospective lover, but Gerald rejected him. Once the Hellfire Club selected Gerald as a new member, Everett knew he could not risk that Gerald would tell the brotherhood what he was – a sodomite – and he decided to eliminate him as a liability. Because Grey had been investigating Gerald's death and now knew the truth, Everett admits that he cannot let Grey live, either. Everett explains that the broadsheets had nothing to do with Gerald's death; they were just the Club's attempts to agitate Sir Richard. Grey, meanwhile, has been plotting his defense while Everett has been talking, and chooses a glass of wine as his best weapon.
They struggle for a minute, Grey with his broken glass and Everett with a knife, until Grey manages to kick Everett hard enough that he staggers backward, right into Harry Quarry's outstretched sword. Grey is unsure how much Quarry had heard, let alone what he might have made of it.
Beyond the memories of it that he would carry, Lord John Grey would also carry a scar from Everett's blade down the side of his neck.