As Stannis Baratheon's ships sail to King's Landing, Matthos Seaworth assures his father, Davos, that come morning, the elder Seaworth will be Hand of the King. They have more men, and the Lord of Light on their side.
In King's Landing, Shae tries to comfort Tyrion Lannister, who is having trouble sleeping. Tyrion knows that the Lannisters won't survive the defeat, so Shae offers to "make love like it's your last day on earth," just like when they first met. Even more anxious, Queen Cersei secures a vial of nightshade from Grand Maester Pycelle.
In the evening, Bronn joins a group of Lannister soldiers at Littlefinger's brothel for some boisterous drinking. The crowd goes quiet with the arrival of the Hound, who refuses Bronn's offer of hospitality. The two exchange words and dirty looks, but the alarm sounds before the men come to blows.
Tyrion hears the bells from his chambers. His squire Podrick Payne dresses him while Varys shows him the city's secret underground tunnel system. Tyrion tells Varys that he will not run; he's the captain of the ship. Varys warns him that Stannis has taken up with a priestess and that dark magic has brought him to King's Landing. Shrugging off Tyrion's skepticism, Varys tells him that a man like Stannis cannot sit on the throne.
In the Great Hall, Tyrion gives instructions to Bronn: Wait until the ships are in the bay. Off to join the other high-born ladies in Maegor's holdfast, Sansa tells Tyrion she is praying for his safe return-like she's praying for Joffrey's. Joffrey shows off his new Valyrian steel sword, Hearteater, and makes Sansa kiss it for good luck. Unhappy when she insinuates he will not be in the midst of the fighting, Joffrey promises that one day the blade will be coated with Robb Stark's blood.
Sansa questions why Cersei-who clearly hates her-wants her in the holdfast. Cersei, already tipsy, encourages Sansa to drink and tells her that Ser Ilyn is there for the protection of the women. Spotting Sansa praying, Cersei mocks her and complains that she was born a woman, unable to do anything but babysit "frightened hens." She shocks Sansa with her regret that her womanly charms will have no effect on Stannis. "If the city falls, these women will be in for a bit of a rape," she says, including Sansa.
Tyrion quarrels with his nephew about strategy on the battlements. When Stannis' fleet comes into sight, Tyrion readies his archers, but does not give the order to fire. Only one royal ship sails into the harbor, leaking wildfire. Davos orders his sailors to steer clear, but it's too late: Tyrion has already signaled Bronn to fire a flaming arrow into the ship. The bay explodes with green flame; Matthos is among those thrown overboard. Certain that Tyrion has no more wildfire, Stannis orders his men to storm the beach, no matter the cost. The Hound goes to meet them directly, but he warns the archers to keep him away from their flaming arrows.
Still drinking, Cersei tells Sansa she hated being married off to a man she never knew. She strikes up a conversation with Shae and correctly identifies her as a low-born Lorathi. Cersei marvels that Shae has managed to become a royal handmaiden in just 10 years time and asks for the details, but Lancel, now injured, arrives to update the Queen on the battle. With the news that Stannis' soldiers are battering the Mud Gate, Cersei demands Lancel bring Joffrey back to his quarters. Turning to Sansa, she confesses that Ser Ilyn is really on hand to see that Stannis doesn't take them alive.
The Hound fights fiercely-and is saved from a hit by Bronn's arrow-but he is dazed by the fires burning around him. Unable to bear it, he abandons his post, despite Tyrion's pleas and Joffrey's orders to return to the battle. Lancel arrives to tell the king that the Queen has commanded him to the Red Keep. Tyrion intercedes and tells Joffrey he needs to lead his men, but Joffrey opts to follow his mother's instructions. Without Joffrey and the Hound, Tyrion picks up the mantle. In a stirring speech to the royal soldiers, Tyrion promises he has a way around Stannis' forces. "We'll come out behind them and f*ck them in their asses," he declares.
Lancel reports to Cersei that the battle is lost-their men lost hope when they saw the king leave the field. When he dares to ask Cersei to let him bring Joffrey back to the action, Cersei pokes his wound. Lancel's groans upset the women in the keep, but Sansa calms them with a hymn. Shae tells Sansa she needs to return to her room and keep her door locked-Stannis won't hurt her; Ser Ilyn will.
In her room, Sansa's is startled to find the Hound waiting for her. He tells her he is leaving and offers to return her to Winterfell, but she decides to take her chances in King's Landing. Tyrion successfully leads the Lannister forces through the tunnels, inspiring chants of "Half-Man." Just when he thinks he is safe, he shares a smile with Ser Mandon, who then tries to kill him. Pod drives a spear through Ser Mandon. Fresh soldiers, including one dressed in Renly Baratheon's armor, take the field. Stannis' men flee at the sight of "Renly's ghost."
Cersei sits with Tommen in the Throne Room and tells him a story about a brave lion cub as she readies nightshade for him to drink. Just as she puts the vial to his lips, the door swings open: Loras Tyrell, dressed in Renly's armor, enters, followed by Ser Tywin Lannister. The Lannisters have won.
Stannis Baratheon's fleet assaults King's Landing in the Battle of Blackwater Bay. Leading the defense, Tyrion Lannister destroys many of the attacking craft with an exploding ship full of wildfire, and is forced to lead a counterattack as King Joffrey and his bodyguard Sandor Clegane each desert the battlefield. Stannis' forces make it inside the castle, but Tyrion leads his men behind the Baratheon forces via underground tunnels and attacks. Meanwhile, Queen Regent Cersei, holed up in the fortress with Sansa Stark and the other ladies of the court, succumbs to wine and despondency as she believes the battle lost. In the nick of time, as she is about to poison her son Tommen, Tywin Lannister's forces, joined by Loras Tyrell, the lover of Renly Baratheon, arrive to dispatch the attackers.
Baratheon's fleet assaults King's Landing. Leading the defense, Tyrion destroys many of the attacking ships with an exploding ship full of wildfire, forcing the former to attack on land. Sandor leads the defense outside the gate, but is defeated and returns inside, denouncing Joffrey and leaving. Joffrey also escapes. Tyrion convinces the Gold Cloaks to fight with him. Baratheon's forces make it inside the castle, but Tyrion leads his men behind the Baratheon forces via underground tunnels and attacks. Cersei goes to the throne room with Tommen, intending to kill him. Shae convinces Sansa to go to her chamber, where Sandor offers to take her back to Winterfell; but she decides to stay. As Tyrion is wounded and Cersei is about to poison Tommen, Tywin's forces, joined by Loras, arrive and defeat Baratheon's forces, with the latter forced to retreat.
At Sea Stannis and his fleet approach King's Landing. Davos' son Matthos thinks the Red God will protect them and when the fight is over his father will be the King's hand. King's Landing Tyrion lies in bed with Shae, worrying that if the city falls, Stannis will burn every Lannister he can find. Shae promises not to let them hurt him, and reveals the dagger strapped to her thigh. Cersei waits for the attack with a glass of wine. Maester Pycelle tries to work up to giving Cersei advice and then presents her with a vial of nightshade, just in case she feels like killing herself. She takes it but sends him away, not wanting to listen to him. Bronn leads a drinking song with the goldcloaks and whores. He undresses one having a grand old time until the Hound comes in, very surly, and giving him the evil eye. They briefly debate the merits of killing. The Hound says Bronn is just like him, only smaller. The Hound stares him down, saying Tyrion will miss him someday. Bronn stands to face him, ready to draw his dagger when the bells toll. Varys brings Tyrion the map he asked for with 50 miles of tunnels beneath the city. He's not planning on running. Varys warns Tyrion about Stannis using the dark arts and that he believes Tyrion is the only man who can stop him. At Sea Davos orders drums to respond to the bells from King's Landing. Joffrey wants Sansa to see him off to war by kissing his new sword that he's named "Hearteater." Sansa asks if he'll be out fighting with the vanguard. He explains that kings don't discuss strategy with stupid girls. She traps him by saying she is being stupid, of course he'll be out there, her brother Robb is always in the thick of things and he's "only a pretender." Sansa's sure Joffrey will be back. "The worst ones always live," she tells Shae. Inside the castle walls, the guards prepare for war and families flee to safety. Joffrey and Tyrion go to the wall and Joffrey is surprised to see his ships are gone. He demands answers, threatening to cut Tyrion in half if he doesn't get them. "That would make me the quarter man, and that just doesn't have the same ring to it," Tyrion replies. He grabs a torch and explains it's all part of the plan. Davos is concerned not to see any ships waiting for them. Sansa and Shae hang out in a bunker for women. Cersei calls Sansa over and offers her wine. Cersei gets word of people stealing horses and orders them killed and their heads put on spikes. She asks for more wine. On the castle wall, they call for the archers as Stannis' ships approach. Joffrey whimpers when they wait to attack. One ship comes out from King's Landing to meet Stannis' fleet. Davos calls for archers and they watch as the ship sails silently past. They see there's no one on board. They see wildfire pouring out of the ship and Davos orders them to steer clear. On the battlement, Hallyne gleefully hands Tyrion a torch; Tyrion drops the torch over the side of the stone wall. From across the cliff, Bronn sees the flame fall from Tyrion's position -- it is a signal and he fires a single flaming arrow into the wildfire by the ships. Davos sees what's about to happen, but it's too late. Matthos ship ignites first, followed by Davos'. The people of King's Landing witness an incredible conflagration as dozens of ships go up in the green flame of wildfire. Tyrion watches in horror as ships break into splinters and men scream in the water. Watching the display, Stannis orders his men forward. He says "the dwarf has played his trick" and "it will only work once." One of his men warns him hundreds will die. "Thousands," Stannis corrects. An increasingly inebriated Cersei calls Sansa over to mock her for praying. She gets Sansa more wine and tells Sansa she should have been born a man, she hates being cooped up. Cersei says that if the city falls, she'll be forced to try to seduce Stannis, but she'd have better chances seducing his horse. She counsels Sansa that her best weapon is between her legs, then warns her that if the city falls, all of the women "should be in for a bit of a rape." Back on the wall, Tyrion orders his men to rain fire on Stannis' approaching fleet. He sends the Hound out to greet them. Joffrey looks generally panicked and twitchy. Stannis' men land on the shore and flaming arrows rain down upon them from the castle. Still, Stannis and some men make it to the castle walls. The King's Landing guards throw rocks over the battlements. Stannis orders his men to the mud gate. The Hound and his men emerge on the shore and fight, Lancel is wounded and retreats for the castle. A drunker Cersei talks to Sansa about Jaime being taught to fight when they were children and her being taught to be lovely and polite before being sold to Robert for him to "ride like a horse." Cersei finally gets a good look at Shae and says that she doesn't recognize her. When Shae approaches and botches a curtsey, Cersei is suspicious and wonders how she got where she is without learning how to curtsy properly. She demands that Shae tell her story, but then Lancel busts in and tells Cersei that Stannis' fleet is destroyed but he still made land. Cersei orders him to bring Joffrey back to his chambers. Cersei fesses up to Sansa that one of the guards, Ser Ilyn Payne, the king's executioner, isn't there to protect them; if Stannis takes the city, he won't be taking them alive. Back outside, the Hound splits men in two and keeps fighting. He's momentarily frozen in his tracks when a man on fire runs at him screaming. Bronn saves him with an arrow to the flaming man. The Hound is totally unsettled by all the fire, has a battlefield epiphany, and leads his men back in the castle walls. Stannis orders the ladders up the walls and goes up one first. He's on the castle wall quickly. Inside, the Hound calls for wine which he chugs. Tyrion yells at him. The Hound tells him he lost half his men. Joffrey bleats at him to go back out. The Hound tells the city, the guard and Joffrey to eff off and walks away. Stannis' men use their upturned landing ships as protection from the flaming arrows and approach the castle walls. They prepare a battering ram. Lancel comes for Joffrey to bring him back to the Red Keep. "If you won't defend your own city, why should they?" Tyrion asks of him.Joffrey asks if his mother had urgent business with him, looking for an out. Then he orders his men to represent him on the field of battle. Tyrion and all the men see him skulk away.His men below observe their King's cowardice and their will to fight diminishes. Tyrion calls for his helmet and shouts that he'll lead the attack. The men need some convincing. Tyrion says there's another way out and he'll show it to them and they'll attack from behind. He tells the soldiers not to fight for their king or for money or anything, but to fight for their city, to protect their houses and their women. "Those are brave men knocking at our door, let's go kill them!" he shouts. The men rally. Not having seen any of this, Lancel reports to Cersei that the battle is lost and that Joffrey should go back out and fight. He tries to talk some sense into her, but she punches him in his arrow wound and storms off. Sansa assures the women that Joffrey will save them and suggests they sing a hymn. Shae tells Sansa to go to her room and bar the door so the guard doesn't kill her. Shae's staying to say good-bye to someone and has her knife strapped to her leg. Back in her chamber, Sansa finds the Hound waiting. He tells her he's going some place that isn't burning. "What about the king?" she asks. "He can die just fine on his own," he says. He offers to take her to Winterfell and promises to keep her safe. When she says Stannis won't hurt her, he tells her he's a killer, as was her father and brothers. She realizes the hound doesn't mean her any harm. Stannis and his men continue attacking the walls and Tyrion and his men sneak out behind them. Tyrion hacks away at soldiers' knees. They fight off the attacking group quickly and start chanting "Half man," but the victory doesn't last long when a huge wave of Stannis' troops approach from another direction. A new clash ensues. Tyrion is attacked by Ser Mandon of Joffrey's Kingsguard and receives a severe slash on his face, but is saved when his squire, Podrick stabs his attacker with a spear. Clearly, either Cersei or Joffrey did not intend for Tyrion to survive this battle. Tyrion slowly loses consciousness as the wound on his face bleeds. Cersei huddles with her young son Tommen on the Iron Throne and tells him the story of a mother and her cub and how she told him that one day everyone will bow to him. Outside on the battle shores, the battle turns when a group of mounted knights seemingly led by Renly Baratheon in his antlered helm and armor crashes into Stannis' flank. Cersei is about to feed Tommen the nightshade when someone bursts through the doors to the throne room. On the wall, Stannis' men drag him away from the lost battle. Twyin strides majestically into the throne room and greets his daughter Cersei. It's clear that the Lannisters had made a pact with House Tyrell as the armored knight removes Renly's helmet and reveals himself to be Ser Loras Tyrell. "The battle is over, we have won," Tywin announces as she lets the vial of deadly nightshade fall and shatter on the throne room floor.