The fourth installment of Taboo opens with James Delaney (Hardy) floating face down in water, reminiscent of the opening credits. This scene proves a kind of meditation for James, as in reality he sits at home before his fireplace, head down and eyes shut. His concentration is broken however by the arrival of the redcoats. Banging on the doors, they have come for Lorna (Buckley).

Taking her to a quiet room, James warns her that these men are from the crown. Their intentions are to force her to relinquish her claim. He instructs her to hold out until the East India Company can release her.

She is whisked away to prison, where Solomon Coop (Watkins) proves himself slimeful in demanding her acquiescence. Shivering from being left half-naked, and tempted by freedom and a thousand pounds, Lorna holds out. And just before Coop could go further, Wilton (Bill) arrives just in time. Brace awaits her with a carriage upon her release.

Strange (Pryce) explodes with fury as Wilton reports that Coop’s papers would have given Nootka Sound to the crown. Recognizing Mr. Delaney’s manipulations and furious at Coop’s treachery, Strange ends the negotiations with the Prince over India.

These events aren’t without consequence for James however. Godfrey (Hogg) reports that the Prince-Regent declined his offer over the pelt monopoly. James’ spy also reports the company’s undocumented discussion over gunpowder, and that the munition is the only thing the Native Americans would want. Unfortunately, the monarchy controls its production for the duration of the war, and now Delaney is blacklisted.

Godfrey encourages James to quit, but James is unrepentant and cruel.

Elsewhere, a chemist named Cholmondeley (Tom Hollander) dazzles a group of women with his pyrotechnics. After the other ladies leave, Cholmondeley grants a… private lesson in more natural chemistry to the remaining, eager girl. Unfortunately, James interrupts the pants-less education with a question of his own. Although understandably frustrated with Delaney at first, the chemist’s choler cools when shown a bulging bag of gold.

James’ return home forces him to kick in his entrance. Brace had taken to setting the house right, and loudly suspects his master knew they were coming. Ignoring him, James visits Lorna in her room. They politely bicker, but she is offered a place in his organization. He needs a trunk of his father’s belongings. But a tired Lorna shows him out of her room so she can rest.

Later that night, James stares out the window of his home where someone watches him. He then performs a ritual over the fire. In the last episode, James promised his sister that they would meet in their dreams. He seemed to have kept his word, as this ceremony effected Zilpha (in her own home and bed). She throws off her blankets in the throes of ecstasy, but the connection breaks when Zilpha (Chaplin) utters a prayer.

The next morning Thorne (Hall) storms into Zilpha’s room, awakening her. Drunk, he mutters some nonsense and makes unwelcome advances. Discovering her arousal, Thorne begins undressing and lays upon her.

The next morning, Lorna throws roses from her room’s window. Likely a signal of some kind.

James escorts Cholmondeley to his new factory. The chemist is hungover from libations the night before: Laughing gas, and a sexually-charged party between higher and lower society. Cholmondeley orally tests components, including wood and… excrement. Between these and a urine, the chemist believes they could make gunpowder in a year.

James doesn’t have that kind of time, but Cholmondeley thinks saltpeter could speed the process. Unfortunately, the only source besides Burma would be in the East India Company warehouses.

As the chemist departs in the back of horse drawn wagon, James’ horse proves reluctant to leave. Taking the animal’s hesitation as a sign, James looks about. He is ambushed by a bald man in a robe, his head cracked by a club. As the bald man drags him away, James knives his legs. After interrogating him with pulp hooks, James cuts him but seemingly lets him live. A scared boy is spotted in the window, likely James’ bastard child from the first episode.

Brace is scared as James sneaks up on him in the manor. Either some time has passed, or James heals unnaturally fast, as he head wound seems gone. With the crown and company checking each other, James no longer worries when Lorna steps out for her performances. But Brace does report his roses were found outside.

Lorna joins them as James berates Brace over reading and resealing a letter addressed to him. Caught, Brace admits it is from Countess Musgrove, a married woman who climbs into beds to climb social ladders. Crazier still, it is an invitation to a ball.

Although there’s another letter from the Americans, James sets it aside to ask Ms. Bow if she dances. Slowly, she smiles.

James visits Winter and Helga. After remarking upon a girl peeing into a bowl, he drops a sack of coins before Helga and instructs her to bring a few girls to the Dolphin. He then visits Atticus (Graham) to ready water transportation for barrels.

And a robbery.

On his way home, James ventures before the closing East India Company Docks, before gathering a satchel from Cholmondeley. The next day, James and Lorna ready for the ball. Taking a coach, James arms one of Helga’s girls with a pistol. He warns Lorna to stay out of “other business” that night.

On arrival, the two exchange quips over the reactions of the other party-goers. Cholmondeley tries to offer some of his gas, but is ignored. In the dancing hall, Zilpha and Thorne’s rhythm is interrupted by James arrival, and his half-sister flees. Unnerved, James steps out to confront her alone. He figures that, as they are both invited, the Americans must know of their relationship. Zilpha knows her midnight wiles are caused by James, and asks him to stop.

Dumbarton (Kelly) interrupts them, and Zilpha flees. The doctor compliments James’ handling of their “first resort,” the bald man that he actually sliced in twain. Dumbarton says the Americans are willing to offer Zilpha as part of the deal, and even handle Thorne for him. He departs, and James checks his watch.

At the East India Company’s dock, Atticus’ man French Bill (Scroobius Pip) grappling hooks a wall. Once over, he lets in Helga’s girl Pearl (Tallulah Rose Haddon) who distracts the guard with a mouthful. Atticus and his cronies handle the patrols through blockades or more violent means. Then Helga steps in with a pistol to relieve her girl’s duties.

For both James and Atticus, all eyes are on the clock. Atticus lets a carriage into the docks, then sets a bomb to blow the lock off the warehouse’s doors.

At the party, James carefully watches both the clock and his sister, and ignores his guest’s questions. But Ms. Bow, in the lady’s room, asks Zilpha about James to stoke jealousy. The actress is silenced when she learns they share the same father.

Lorna approaches James, hoping that he is interested in her. But James instead delivers an ultimatum: deliver the trunk of letters or their association is through. Musgrove (Marina Hands) interrupts to ask a dance of James, to which he responds negatively. He is deeply uncomfortable surrounded by the party goers, and is pulled into a magician’s closet with the countess.

The closet’s false wall flips to give them solitude. James reveals he knows she is truly from New Orleans. And that she “sent the child to kill” him, but the circumstances changed her mind. She is Carlsbad. After a brief interruption, he confirms he wishes to meet with the embassy in Paris, despite warnings against it.

The party is now at peak debauchery. Thorne, quite inebriated, attempts to insult James. But the latter grabs his brother-in-law and throws him out into the yard. Embarrassed before the on-lookers, Thorne challenges James to a duel to the death.