After being engulfed in the white light upon the Other Side's destruction, Bonnie and Damon found themselves in what appeared to be a parallel world of Mystic Falls. They soon realized that they were in a different time, due to all the old cars parked on the streets that appeared to still be in nearly brand-new condition. After finding a newspaper in a nearby yard, Damon discovered the date on the paper was May 10, 1994. They later found out that the dimension they were in was stuck in a temporal loop, as the solar eclipse they experienced on their first day occurred at the same time each day afterward. However, they were aware of the time loop, and maintained their memories of the previous day.
In Yellow Ledbetter, Bonnie and Damon continued living together in the Salvatore Boarding House while they tried to figure out how they got to the prison dimension and how they could possibly get out, which led Bonnie to track down her Grams' grimoire in the hope that she could re-learn magic and use it to free them. Later that evening, the crossword puzzle Bonnie had been working on all day was mysteriously filled out by someone else, and Bonnie became convinced that they were not alone in this world.
In Welcome to Paradise, Bonnie and Damon argued amongst themselves about whether or not they were not the only ones trapped in this world as they came upon several clues that someone else was in their vicinity. After Bonnie and Damon got into an intense fight, Bonnie stormed off, and Damon went into Bell's Store to get some bourbon, where he finally came upon the person following them. It was a young man who identified himself as Kai, and who tricked Damon into consuming vervain. Kai was about to try to kill him when Bonnie arrived, and she was so worried about Damon that she managed to finally reactivate her magic and use pyrokinesis to create a boundary of flames around Kai to trap him. Damon then took the opportunity to knock Kai out, and they brought him back to the boarding house to question him about why he was in this dimension. After some threats on Damon's part, Kai finally admitted that he never intended to kill Damon -- he was merely trying to motivate Bonnie to get her magic back, as it was the key to returning them to their home dimension.
In Black Hole Sun, Bonnie and Damon continued questioning Kai, who informed them that they would need to use Bonnie's magic to harness the power of the solar eclipse using a mystical object belonging to his family called the Ascendant. After doing the requisite spells to prepare for their return home, the three went outside so Kai could figure out exactly where the power of the eclipse would be focused for their portal. While they were watching Kai work, Bonnie re-read their newspaper and realized that one of the main articles on the front page was a story about a young man named Malachai who killed most of his siblings in Portland, Oregon. Kai freely admitted that it was he who killed his siblings, and even confessed that their dimension was a Prison World created for him on May 10, 1994 by the Gemini Coven as punishment for his crimes. Bonnie, horrified by this story, refused to do the spell to return them home, even despite Damon's pleas that they needed to return to their loved ones. Later that night, Damon and Bonnie had dinner together, where Damon suggested that they steal the Ascendant and go home by themselves. However, they were interrupted by Kai, who came in and informed them of something he didn't mention earlier -- while he is a witch, he didn't possess his own magic, and instead could only siphon it from other witches to use for his own means. He then grabbed Bonnie by the arm and absorbed a portion of her magic before giving her and Damon an ultimatum: he could steal all of Bonnie's magic to the point of her death, kill Damon, and then use the Ascendant to go home alone, or they could all go home together as friends. This forced Bonnie and Damon to begrudgingly agree to his terms in order to ensure their survival.
In The World Has Turned and Left Me Here, as Kai, Bonnie, and Damon were preparing to do the spell the following day, Bonnie realized that the reason why Kai hadn't killed her was because he needed a Bennett witch specifically to do the spell. Since Kai was no longer needed, she telekinetically threw an axe into Kai's chest, and he fell to the ground, seemingly dead. However, before Bonnie and Damon could leave, Kai was revived and interrupted them, using a crossbow to shoot Bonnie in the stomach with an arrow in the process. He asked them if they really believed he hadn't tried to kill himself, and added that he had attempted in many times in many ways, but as the Prison World was meant to be his punishment, he was unable to die in it. He tried his best to prevent them from completing the spell, but Bonnie was able to magically throw Kai backward, allowing her to pull Damon into the light of the eclipse and throw him the Ascendant so he could leave and return to the real world in Mystic Falls, believing that she would die before she could get there.
In Do You Remember the First Time?, Bonnie awakened in the Salvatore Boarding House with Kai, who was trying to reassemble the Ascendant. She temporarily neutralized him by killing him before she fled to the hospital to heal her injuries. She also rebuilt the Ascendant from the pieces she had stolen before she left, but she had forgotten one piece, and Kai revived himself and got to her before she could finish it and escape. He then forced her to do the spell again to get them out of the Prison World. However, when the two went into the tunnels, Bonnie tricked Kai by putting her magic in her teddy bear, Ms. Cuddles, and sent the bear and her magic to the living world so Kai couldn't absorb it from her and use it to escape. Kai and Bonnie were then left in the Prison World, still trapped and now magic-less as well.
In Fade Into You, Kai took Bonnie to Portland for Thanksgiving to have dinner at his family home, but eventually revealed that he had brought her there because he needed magic, and Bonnie's trick with her teddy bear made him realize that his older sister Jo had hidden her own magic in the hunting knife with which he had stabbed her in 1994. Bonnie argued that he still needed a Bennett witch to do the spell, but Kai corrected her and told her that all he needed was Bennett blood. He then took the magic from the knife and absorbed it before stabbing Bonnie in the stomach with it. With Kai now in possession of the Ascendant, Bonnie's blood, and the magic from the knife, he was able to ditch her in his yard and, at some point, escaped the Prison World, leaving Bonnie dying and without magic in Portland. Meanwhile, in the real world, two events shed light on the Gemini Coven and Kai's imprisonment; one was the Friendsgiving dinner at Elena and Caroline's dorm, attended by Jo, Liv, and Luke (as well as Tyler and Liam) and in Portland at Kai's family's home, where Damon, Stefan, and Alaric had traveled to meet the leader of the Gemini Coven, Joshua Parker. It was revealed that Jo and Kai were twins, and that they were the siblings of Liv and Luke, as well as the children of Joshua Parker and members of the Gemini Coven. Since in their coven's ancient tradition, leadership is determined by a ceremony called the Merge, in which a pair of twins in the coven casts a spell in which the stronger witch of the pair absorbs the magic of the weaker witch, essentially killing the weaker twin and doubling the strength of the stronger one. When it was learned that Kai did not possess magic of his own, and only had the ability to siphon it from others, Joshua and his wife, realizing Kai wasn't fit to lead the coven, continued having more children until they had a second pair of twins, Liv and Luke, who they planned to have lead the coven instead. When Kai learned about this, he went on a rampage to kill Liv and Luke to ensure that he and Jo would be completing the Merge instead, killing four of their siblings to get to the younger twins and seriously injuring Jo in the process. To save the twins, Jo agreed to do the Merge with Kai and to let him kill her to take her power and leadership of the coven, but it was ultimately revealed to be a trick-- since Jo had put her magic in the knife, Joshua and the Gemini Coven were able to overpower Kai, using a Bennett spell and the solar eclipse to lock Kai away in a Prison World for his atrocities against their coven.
In I Alone, Liv used the Ascendant from their dimension to send Damon and Elena into the Prison World to get Bonnie back, not knowing when she did the spell that Kai had already escaped his prison. As Bonnie was still in Portland and not in Mystic Falls where Elena and Damon had arrived, the two paged Kai's pager to get a hold of her on a payphone to tell her the plan, and Bonnie hot-wired a car to get back to Mystic Falls and meet them at the Gilbert House. However, right before Bonnie returned to their meeting place, Liv, having been confronted by Kai at Scull Bar, was forced to pull Elena and Damon out of the Prison World early, not wanting them to be trapped if Kai hurt or killed her while they were there.
In The Day I Tried To Live, Bonnie, having spent nearly an entire year in the Prison World, and incredibly depressed to have to spend her birthday alone without any company whatsoever, decided she wanted to commit suicide. However Kai, who had just completed the Merge with his brother Luke and became the leader of the Gemini Coven, was feeling so remorseful about his actions after having absorbed Luke's empathy that he agreed to astral-project himself, Jeremy, Elena and Damon to the Prison World to try to get Bonnie out again. This plan ended up not working as intended, as the amount of power required to send all of them to the Prison World left them unable to alert Bonnie to their presence; fortunately, Damon managed to leave a note on the atlas pointing Bonnie to the Island off of Nova Scotia where Silas' headstone (which was full of Qetsiyah's magic-infused blood) was located. From there, all they needed to do was ensure that Bonnie would find the clue before she did anything drastic. However, they ran into further difficulties when Liv showed up, intent on killing Kai to avenge Luke's death even despite knowing that killing the leader of the Gemini Coven would result in the death of everyone in the coven, including herself, her father, and her sister Jo. Jeremy managed to save the gravely injured Kai and convince him to send the two of them back to the Prison World to try again while Damon saved Kai with his vampire blood. Jeremy came upon Bonnie attempting to kill herself by locking herself in the garage and suffocating herself with the fumes from Damon's Camaro, but when Bonnie was leaving her goodbye video message to everyone, she remembered Grams' parting words to her before she found peace -- "Stay strong." Too weakened to leave the garage, Jeremy began to panic, eventually getting so upset he was able to make contact with the button to the garage door, giving Bonnie fresh oxygen and saving her from dying. When she returned to the house, she found the clue left by Damon in the atlas, which including pertinent information such as the low tide of the ocean surrounding the island, and, renewed with hope, Bonnie took off for Canada in Damon's car to get the magic she needed to escape the Prison World.
In Let Her Go, Bonnie returned to Mystic Falls from Nova Scotia with her magic renewed thanks to Qetsiyah's blood. However, in the real world, Kai's incomplete Merge ceremony with Luke had caused several side effects and was making him, in his words, "magically diseased." When Jo and Alaric were reluctant to help them after Kai realized he needed Jo's magic to fully complete the Merge, he revealed that the spells that created the Prison Worlds were bound by the magic of the leader of the Gemini Coven, and that if he died, the worlds would collapse. Shocked and confused at the realization that there was more than one Prison World and not wanting the rest of the Gemini Coven to die along with Kai, Jo agreed to give up her magic and lose her status as a witch to complete the Merge. However, since Kai was bordering death prior to and during Jo's spell to give him her magic, the 1994 Prison world collided with another Prison World just as Bonnie was preparing to escape and return to her original dimension; instead of a spring day during a solar eclipse, she was suddenly in a winter night during an Aurora Borealis. Bonnie, extremely confused and scared, saw footsteps leading to the Salvatore Boarding House and followed them into the home, quickly realizing by the lack of electricity inside that something was seriously wrong. It was then that she found a journal, dated November 1, 1903, that discussed the power of the recurring Aurora Borealis in the dimension's time-loop, as well as a framed pair of photos of both Damon and Stefan from 1864. Just as Kai was being given Jo's magic in the real world, the Prison Worlds began colliding again, and Bonnie rushed back into the tunnels as her surroundings cut back and forth between the warm sunshine of spring 1994 and the cold snow of winter 1903. Realizing that the Northern Lights was a celestial event she could draw upon just like the solar eclipse in order to cast the spell, she sliced her palm with the Ascendant and stated the incantation to create the portal. Just before the portal to the real world opened up, Bonnie was suddenly approached by a woman with auburn hair and blue eyes, who looked confused and asked Bonnie who she was. Bonnie repeated the question, but before she could get an answer, she was transported back to the real world. Once she had reunited with Damon, she showed him the video she had unknowingly recorded as she was sucked into the portal, and Damon, completely shocked at what he saw, confessed that the woman was his mother, Lily Salvatore.
In I'm Thinking Of You All The While, Kai, the leader of the Gemini Coven, dies and the prison worlds collapsed.