Since the start of building the 17th Century, and through its completion in the 18th, the Fort of San Cristobal defended Puerto Rico from different invasions, including the Dutch, the English and the Americans during different invasions.
Over time, one of its sentry towers developed a legend of being haunted. According to the legend, the tower, named "La Garita del Diablo" or The Devil's Sentry, was the last known location of many guards that disappeared in the middle of night, never to be seen again.
Search for the City
Infiltration into Castillo San Cristóbal
Bobbi Morse and Phil Coulson discussed how to enter the Castillo San Cristóbal, to the only access point to the hidden city that was not under the sea. Morse and Coulson consulted a series of plans that Diego obtained from the city archives, that were older than the city of San Juan itself.
Coulson and Morse went to one of highest points of the fort to compare the map with the satellite spectrometry of the hidden city, and figured out that the vertical shaft that would serve as an access point was located in the deepest part of the fort, just below a garrison in a sentry tower inaccessible to the public.
Coulson ordered Morse to contact Diego again, in order to help them get their equipment to the garrison. Morse asked Coulson about his theory of how HYDRA planned to release the true power inside the Obelisk, given that nobody could even touch the artifact, but Coulson acknowledged that maybe somebody was able to touch it.
Phil Coulson received a call from Melinda May at the Bus, who informed him that the extraction mission to retrieve Raina from Vancouver was successful, as HYDRA did not send a big contingent to kidnap Raina, despite Agent 33 took point of their operation.
May revealed their theory for HYDRA's attempt to kidnap Raina, as according to her, she could touch the Obelisk without being harmed, and would lead its holder to the temple. Skye believed Raina, and was open to reveal her many details, just as Coulson had predicted. Coulson asked May to make sure that Skye continued to talk to Raina, in order to obtain any valuable information about the temple, and ended the conversation.
While Coulson was talking to May, Bobbi Morse tried to make Diego help them, but he refused to descend to the garrison, as he was afraid of a local legend about it, that had named the place as "La Garita del Diablo" or The Devil's Sentry.
La Garita del Diablo
Alphonso Mackenzie, Leo Fitz and Jemma Simmons descended to the garrison of the Castillo San Cristóbal in order to open an access point to the hidden city; Simmons began commenting about the legend of "La Garita del Diablo" or The Devil's Sentry. Simmons explained that while the whole fort was built in the 18th Century, that tower was built in 1634, and since then it had the fame of being haunted.
Mackenzie used a Mouse Hole to open an entrance to the hidden city, revealing a deep hole to the surface. Mackenzie exclaimed he was not scared of ghosts, but asked Simmons for clarification about the legend. Simmons told that, according to the legend, several of the guards in the sentry tower went missing never to be seen again. Simmons claimed the legend seemed ridiculous, though Mackenzie compared it to an extraterrestrial city.
Phil Coulson and Bobbi Morse arrived at the garrison to check their progress. Fitz and Simmons were unable to decide which of them should answer, making Coulson tell them to start talking, no matter which one of them. Fitz said that they were ready to deploy the D.W.A.R.F.s, and activated three of them to descend through the hole.
Coulson asked about the time it would take to fully survey the tunnel and locate the temple inside the city, and Fitz tried to explain that it would depend on several factors, but was unable to find the appropriate words. Mackenzie, who usually helped him in these cases, turned a blind eye to make Fitz request Simmons' help.
Simmons helped Fitz, asking if he wanted to speak about the tectonic stress levels, and then again about the structural integrity of the tunnels. Mackenzie guessed the temple would be distinctive enough to recognize it upon seeing it, but Simmons explained that they depended on the data sent by the D.W.A.R.F.s.
Suddenly, the D.W.A.R.F.s got disconnected inside the city, and Fitz determined that since the tablet still worked, the D.W.A.R.F.s were the ones that were disconnected upon arriving.
Sentry of the City
Alphonso Mackenzie prepared to descend towards the hidden city under Castillo San Cristóbal, through the hole of an estimated height of 100 feet. Mackenzie was not scared of heights, though he could not say the same about the dark.
Phil Coulson handed Mackenzie a handgun, just in case, and asked them to fix the D.W.A.R.F.s, that were previously sent through the hole and went offline. Leo Fitz theorized that all electronics could have been jammed inside the city. Coulson told Mackenzie to make three pulls on the cable if he needed help, and Fitz started to Mackenzie down the hole.
Mackenzie reached the bottom of the hole, and though he could not use the radio to contact, he yelled at the hole, obtaining an answer. Mackenzie noticed the D.W.A.R.F.s were on the ground, and touched the symbols at the entrance. This triggered a reaction that slowly transformed Mackenzie into a guardian for the city, starting with displaying the Words of Creation in his hand.
Hearing Mackenzie's screams of pain, Coulson ordered to lift him. Worried, Jemma Simmons explained that the D.W.A.R.F.s did not read any sign of life in the city. Once he was finally lifted, Mackenzie was removed from the harness while he continued to scream and tremble.
Bobbi Morse, extremely worried, tried to make Mackenzie talk to her, while Simmons quickly examined him searching for wounds or signs of blood, to no avail. Coulson approached Mackenzie once he stopped screaming who, with his last remaining strength, told him to run before succumbing to the transformation.
Mackenzie's eyes turned red and he started attacking his friends with superhuman strength. Morse tried to reason with Mackenzie while dodging his blows. Morse retaliated, but her attacks were ineffective, so she dodged Mackenzie again to retrieve her battle staves.
Mackenzie grabbed Morse and began to strangle her. Meanwhile, Fitz threw an I.C.E.R. to Coulson, who promptly shot Mackenzie, but the bullets did not affect him. Morse tried to reason with him, but he was only distracted as Coulson grabbed his own neck.
Simmons then attacked him with a suitcase, and Mackenzie's blow almost made her fall through the whole to a certain death. Coulson was able to catch her, while Fitz retrieved a handgun and pointed at him. Fitz tried to reason with him again, as his handgun was real and would kill him. Coulson ordered Fitz to shoot, but Morse used her battle staves to electrocute Mackenzie, seemingly knocking him out.
Mackenzie fell through the hole, and Coulson ordered to seal the tunnel. Morse, in tears, asked what would happen to Mackenzie, but Coulson replied that was not Mackenzie anymore.
Setting the Bombs
Antoine Triplett, Leo Fitz and Jemma Simmons prepared inside Castillo San Cristóbal to infiltrate the hidden city. Triplett told Fitz and Simmons that one of them should stay out of the city in case anything went wrong, but both of them explained that given electronics would be inoperative in the city, the had higher probability rate of success working together.
Triplett checked the timers for the explosives, and all that remain was place them in the chosen spots to destroy the temple, insert the detonators and escape from the city.
Simmons asked what would happen if Alphonso Mackenzie was inside the city, and Triplett explained the two options. If Mackenzie was dead, they would bring his corpse outside, but if he was not dead, that would mean he was not Alphonso Mackenzie anymore, and he would be forced to kill him.
Triplett was the first to descend through the hole, but Mackenzie was nowhere to be seen, and Triplett wondered where would he went.
Antoine Triplett, Jemma Simmons and Leo Fitz emerged from the hidden city through the entrance under the Castillo San Cristóbal, ten minutes before the detonation of the bombs. Fitz was worried that Alphonso Mackenzie was still inside the city, and would die or at least be trapped in the explosions.
Melinda May called Triplett, who informed her that they had managed to place the explosives. However, May said Raina had already entered the city with the Obelisk. Triplett was willing to let Raina die in the explosion, but as May informed that both Skye and Phil Coulson were in the city too, Triplett entered the city again, despite Fitz and Simmons' attempts to dissuade him.
Jemma Simmons exited the Kree City and called the Playground from the Castillo San Cristóbal to inform Phil Coulson and Melinda May that they had been attacked by Raina. Coulson asked how many victims there were. Simmons stated that Raina attacked five agents; two were dead and three were in critical condition.
May wondered how Raina could have done that without a weapon, but Simmons explained that her body was covered in thorns, something very hard to comprehend. Simmons also informed that she shot Raina a few times, but she was unable to kill her, though she obtained tissue and blood samples.
Coulson asked how long would it take to blow a vertical shaft on the ocean floor and drown the city. She informed him that her team was preparing the charges, so the city would be underwater and sealed in no time.
Simmons requested to return to the Playground instead of joining the manhunt for Raina, in order to study Raina's samples and check on Skye, as she was exposed to the same substance that transformed Raina. Coulson agreed, saying that they had enough men in Puerto Rico to chase Raina, and prompted Simmons to return home.