The Imprisonment in Antarctica was an unsuccessful operation by the Agency to imprison Cipher and Letty Ortiz in a blacksite prison.
Background
Following an attack by Dante Reyes which resulted in her syndicate turning on her, Cipher approaches Dominic Toretto and Letty Ortiz warning them of Dante's impending revenge plot. After contacting Mr. Nobody, the Agency arrived at 1327 and apprehended Cipher, taking her to Blacksite Prison in Antarctica.
After Dominic Toretto's Crew were framed for a terrorist Attack in Rome, Letty is apprehended by the police who hand her over to the Agency where Aimes sends her to the same black site prison as Cipher.
Imprisonment
Tess approaches Letty in the prison and the two orchestrate an attack that leads to Letty being transferred to the same medical facility that Cipher is behind held in so they can work together to escape. Cipher reveals that she re-routed the ventilation system in the medical facility to blow knockout gas onto the medical personnel, giving Letty and Cipher a chance to plan an escape.
Cipher reveals that they can escape through the vents but will have to wait around four minutes for the gas to clear up so they can safely travel through the vents. Letty says that's enough time and she proceeds to fight Cipher in the facility, taking her down and getting vengeance for Cipher's previous attacks on them.
Following the fight, Letty climbed through the ventilation system and found an exit hatch and discovered that the prison was located in the middle of Antarctica. After climbing back down, she was given a snowsuit by Cipher who revealed she already knew where they were. The two escaped through the vents and began trekking through Antarctica, with Cipher getting Letty to trust her.
Aftermath
As they trekked through Antarctica, Letty and Cipher were climbing over a mountain and witnessed as a submarine broke through to the surface. A woman emerged from the submarine watching them through binoculars and after lowering the binoculars was revealed to be Gisele Yashar who had survived her fall during the Runway Chase. Cipher then exclaimed to Letty "still think my plan sucks?" revealing that she and Gisele may have acquainted with each other in the past.