Lockdown of the Men of Letters bunker

The Lockdown of the Men of Letters bunker was an effort by British Men of Letters operative Arthur Ketch to kill Sam and Dean Winchester alongside Lady Toni Bevell by trapping them in the Men of Letters bunker to suffocate.

Background
Shortly after the Darkness left the Earth, the British Men of Letters made their presence known in America. Led by Mick Davies, the organization began attempting to recruit American hunters to create "a world without monsters" without much success. Mick eventually succeeded in recruiting Mary Winchester, but otherwise failed to recruit "the top-shelf" of American hunters, only getting hunters such as Pierce Moncrieff.

Following the attack by the Alpha Vampire, Sam Winchester agreed to join the British operation, believing that they were doing a great deal of good in the world. He eventually convinced his brother Dean to sign up as well. However, after learning of the existence of a Nephilim, British Men of Letters Elder Doctor Hess ordered Mick Davies to "assimilate or eliminate" the American hunters. After a failed attempt to kill Dagon resulted in the death of British operative Renny Rawlings, Doctor Hess had Mick executed by Arthur Ketch for not following the British Men of Letters code by killing Eileen Leahy and ordered the American hunters wiped out.

As a first step towards this goal, Arthur tricked the Winchesters out of their bunker so that he could set up surveillance upon them. Despite Arthur's attempts to hide his work, the preparing operation to wipe out the American hunters is eventually discovered by Mary Winchester along with the body of Mick Davies. She is captured by Arthur and subjected to extensive brainwashing by Lady Toni Bevell that ultimately leaves Mary under the control of programing that renders Mary completely loyal to the British Men of Letters and their orders. In this state, she is sent to assassinate fellow hunters.

The British Men of Letters eventually begin an operation to kill all of the American hunters by, alongside using the brainwashed Mary, getting a hellhound from Crowley. Using this hellhound, Arthur kills seven hunters, including Eileen Leahy. The type of deaths, along with the lack of communication from their mother, draw the suspicions of the Winchesters. After receiving a letter from Eileen in which she states her belief that the British Men of Letters are following her and tapping her communications, the Winchesters discover Arthur's surveillance equipment and use it to draw Toni into a trap and capture her. Toni confirms their suspicions about the British Men of Letters activities while in the Winchesters' custody.

Battle
Returning to the Men of Letters bunker, Dean tells Lady Toni Bevell that they will call Arthur Ketch and tell him that if he ever wants to see her again, he will come to the bunker. However, they find Arthur already waiting for them with an assault team.

Arthur orders Toni to disarm the Winchesters, but as she grabs Sam's gun, Sam pulls her to him and shoots one of the assault team members in the head before he can react. Dean quickly shoots another from behind his back and the Winchesters, Arthur and the surviving assault team member begin exchanging fire with Sam using Toni as a human shield.

Spotting Arthur's surviving man leaving to circle around behind Sam using the bunker's hallways, Sam signals Dean to go after him while Sam continues to exchange fire with Arthur. Moving through the bunker hallways, Dean finds the remaining assault team member and quickly kills him. As Arthur continues to exchange fire with Sam, Dean comes up behind him, snatches Arthur's gun from his hand and holds Arthur at gunpoint, ending the gun battle.

As the Winchesters hold Arthur at gunpoint and demand to know who else is in the bunker, Mary enters and orders her sons not to move, now fully brainwashed to the British Men of Letters side. As Dean stares at her in shock, Mary fires a shot at the pillar next to him, causing Dean to duck and allowing Arthur to snatch Dean's gun from him. Armed with both Dean's gun and his own, Arthur takes aim at the two Winchesters and warns them not to move as Mary really will shoot them.

Mary takes Sam's gun from him and begins backing towards the door with Arthur, both holding the Winchesters at gunpoint. Toni tells the Winchesters that Mary was always a talented hunter, but always confused about obeying orders. After Sam demands to know what the British Men of Letters did to Mary, Arthur informs them that "Lady Bevell cleared up that confusion" and that "the American hunters are a dying breed." Toni attempts to leave with Arthur and Mary, but is prevented from going by Arthur who sees her as a rival and wants her dead. Dean calls out to Mary for help, but her brainwashing proves to be too strong and she won't help them. Arthur informs the Winchesters that he has shut off the water to the bunker, rejiggered the locks and once he and Mary are gone, the pumps that bring air in will reverse. As a result, the Winchesters and Toni will be trapped with no water and a depleting air supply, leading to their eventual suffocation in two to three days. Arthur and Mary depart, locking the door behind them. Moments later, the lockdown set up by Arthur initiates, causing the power to go out and the air pumps to deactivate.

After Arthur leaves, the Winchesters try without luck and begin discussing their options for escape. Toni shoots down the ideas of using the garage and air vents and Dean suggests killing Toni to conserve air. Toni tells the Winchesters that if they ever get out, Toni is the one who brainwashed their mother and thus is the only one who can undo it. The Winchesters reluctantly concede that they may still need Toni and question how they can get out. Toni informs them that the only way out is to hit the bunker's manual override, but its outside the bunker where they can't reach it. Sam realizes that they are in a giant vault full of occult books and lore and suggests searching for an object or spell that can reverse the lockdown and get them out.

After a day of searching through the bunker's archives, Sam finds the Abrogation Ritual, a ritual used by the Romani people to reset machinery. However, while they possess most of the needed ingredients, the ritual requires that the mechanisms be anointed with the blood of virgins which none of them are. At Sam's suggestion, the Winchesters and Toni use a Blood Purification Ritual to "revirginize" their blood for the ritual. Toni is successfully able to perform the ritual and it appears to work at first, the power returning and the lights returning to their normal colors. However, the ritual ultimately fails and Toni realizes that Arthur has added a mystical dampener to the lockdown, preventing magic from working.

After another day of fruitless searching, Dean comes up with a plan to use brawn instead of brains to escape. Examining a blueprint of the bunker, Dean discovers that while most of the walls are reinforced steel, there is one that's just concrete and has an old sewer pipe on the other side. Dean suggests breaking through the wall into the sewer pipe and using it to reach the manual override. Sam and Dean begin attempting to break through the wall with pickaxes, but barely make any progress and are forced to stop due to exhaustion and the thinning air. The Winchesters realize that they won't hit dirt for at least a few days and that they are almost out of air and won't make it that long.

As they discuss their situation and how they thought they'd die, Dean comes up with another idea to get them out: use his grenade launcher to blow a hole in the wall. Toni protests as the explosion could bring the whole bunker down on them, but the Winchesters are undeterred. Once Sam and Toni have taken cover, Dean blasts the wall his grenade launcher. Despite injuring his leg, Dean succeeds in blowing a hole in the wall without bringing the bunker down on them. Dean is able to make it out of the bunker through the sewer pipe and hit the manual override.

After Dean blows a hole in the wall, Sam checks to find the hole and so sign of Dean. The hole collapses moments later. As Sam makes his way back to the library calling for Dean, the oxygen runs out and Sam begins to suffocate. Before Sam can die, Dean hits the manual override and the lockdown ends, restoring the power, water and air pumps. As air returns to the bunker, a triumphant and injured Dean returns through the main entrance to greet Sam.

Aftermath
Following their successful escape from the bunker, the Winchesters begin contacting other hunters and warning them of the threat from the British Men of Letters. Upon contacting Jody Mills, Sam learns that Jody and Alex have managed to capture Mary. Taking Lady Toni Bevell to Sioux Falls, the Winchesters learn that she lied about being able to break Mary's brainwashing so they wouldn't kill her. While Dean returns to the bunker with Toni and Mary to save his mother, the threat to the American hunters causes Sam to organize an attack by the American hunters on the British Men of Letters compound to stop them once and for all.