The Islands are a small cluster of Islands to the southwest of Ooo inhabited by Humans. After the Mushroom War, a group of human survivors (also known as the Founders) left what later became Ooo due to the mutants and magical beings that made it a dangerous environment to live in. The humans then came upon the four islands and settled there. They also built a Guardian to prevent any monsters from coming to the islands and preventing any of the island inhabitants from leaving without being killed by it. The Islands are the birthplace of Finn and most other humans seen in the show.
There are four islands in the archipelago. The list of islands is:
Before events of the Main series and Islands miniseries, The Islands government closely resembled a Meritocratic-based Technocracy rule by few highly intellectual authoritative figures (Helpers) to control all colonies.
By the time of the events in the Island miniseries, all except Founders Island are ruled by no one due to Dr. Gross's deadly virus wiping out all the Helpers including Minerva before giving up her physical body to save herself by digitalized means. Minerva resumed control of the last remaining island as the only Helper left under Dictatorship-like authority through the use of Minerva Bots stationed Island-wide.
In the graphic novel "Adventure Time: Islands" it was revealed that Two Bread Tom led his tribe to salvation on the ship they had managed to repair and escape from the monster-infested mainland on. They sailed for 34 days and eventually found an island that would eventually become known as Founders' Island. The tribe united with a second group of humans who had already been living there, and they begin to build a settlement. Tom had become an important role model to the people of the island and became the first of the Islands' legendary Founders. After Jo was abducted by a giant bird, he learned that the island was more dangerous than expected and became obsessed with protecting his people. He began construction on a large wooden sentinel, probably the precursor to the Guardian. Jo and several other hiders disagreed with his plan to completely seal off the island, so they split from the main group and became the first hiders.
After many generations, the human race continued to exist a thousand years later but had not ventured beyond the islands. They developed their technology to beyond pre-war levels, reinventing cars, robotics, advanced healthcare, virtual reality, and much more. They colonized two other islands in the chain: Hub Island and the unnamed Better Reality Island, where humans could retreat into a virtual world.
As seen in "Mysterious Island," at some point an expedition was sent to the fourth and final island, Back-to-Nature Island, in an attempt to colonize it. They were partially successful and managed to establish a small settlement and advanced artificial weather systems. However, almost everyone on the expedition was evacuated or killed by either the malfunctioning weather system or the over-sized wildlife, with the lone exception of Alva who seems to be the only human still living there.
The humans (on Hub Island at least) are assigned a function from early childhood and train for most of their lives. Some of these careers include helpers (doctors and nurses), seekers (people who would prevent people from leaving the island), and experimenters (people who make gadgets and toys). Most humans chose to follow the Founders' plan, which meant staying on the islands away from the danger of the outside world. Those who didn't were known as "hiders."
It was revealed in "Hide and Seek" that Kara Strong was a seeker raised on Hub Island. Her best friend was Frieda, an experimenter who turned out to be a hider. Kara tried to help her escape, but Dr. Gross used Kara's implant to control her mind and force her to capture Frieda.
It was revealed in "Min and Marty" that 12 years before the start of the series, Finn was born on Hub Island to his father Martin Mertens and his mother Minerva Campbell. Martin was a con-artist while Minerva was a helper, but the unlikely couple fell in love after Minerva helped Martin escape re-education when he was mistaken for a hider.
One night when Minerva was at work and Martin was left alone with baby Finn, an old foe of Martin, Widow, broke into their home and forced Martin and Finn to escape onto a raft in the ocean. Martin only intended to circle around the island and return, but the Guardian mistakenly believed that they were trying to escape, and attacked their raft. Finn and his father were separated but did make it alive past the Guardian, possibly becoming the first to do so. After this, Dr. Gross dispatched Kara, her most powerful seeker, on a transport ship to go to Ooo and find them both, but she wouldn't return until the events of the "Islands" mini-series.
As seen in "Helpers," at some point after Martin and Finn disappeared, Dr. Gross accidentally released a deadly virus on Hub Island by a Rat-reptile. 62% of the population died from it, including nearly 100% of the helpers who were regularly exposed to the disease. Minerva (who was possibly the last helper) gave up on resisting the disease in her body and uploaded her consciousness to the web, becoming an A.I. With this, she had the power to be hundred of helpers in robotic bodies. Through quarantine and treatment, she saved the remaining humans on the island. The survivors were quarantined on Founders' Island, leaving Hub Island abandoned. Dr. Gross fled the islands in her mobile laboratory and would not be seen again until the events of "Preboot."
In "The Invitation," a ship from Hub Island comes to collect Susan (Kara), she, Finn, Jake and BMO embarked on a voyage to find the humans. They explored each island in turn.
In "Mysterious Island," they wash up on Back-to-Nature island, where Finn meets Alva and learns about the failure of the expedition. She is living in a teched-out tree base by this time along with her pet giant bear Albert.
In "Imaginary Resources," Finn, Jake and BMO visit the unnamed Better Reality Island. They discover a large number of humans sealed away in pods, living contentedly in a virtual world. They are fed and kept alive by an army of small service robots. Jake destroys the computer equipment only to find that the humans would rather be living in their virtual world, so he lets the service robots repair everything again.
From there, they climb into a transport tube and arrive on Hub Island in "Hide and Seek," where they meet back up with Susan. Susan wanders through the abandoned wreckage of the island and reminisces about her past, remembering her past identity as Kara. After that they take a transport and fly to Founders' Island. Finn learns that his mother is still living on the island in "Min and Marty."
In "Helpers," the group arrives on Founders' Island, which is still populated by humans. Finn is surprised to find his mother within only a few minutes of arriving, but quickly discovers her true nature after Jake's powers begin to scare the citizens. Minerva bots arrive at the scene and abduct Finn and Jake, and Finn finally meets his mother, who is still uploaded onto a computer. For his safety, Minerva refuses to let Finn leave the island, wanting him to upload his mind to the web.
In "The Light Cloud," Finn goes back down into the streets and convinced many of the humans that the world wasn't such a bad place. Minerva saw this and was surprised that people were actually listening to him, so in a rash decision she planned to upload everybody's brain onto island's web. Finn then showed Minerva his memories of helping people all over Ooo. After learning that Ooo taught Finn to behave like that, Minerva decided that it may not all be bad. Minerva then allowed everyone to leave the islands if they wanted to and deactivated the Guardian after nearly a thousand years of service. However, no one except for Susan's friend Frieda ended up leaving because it was "too early" for most people.
- Wolves (on Hub Island, possibly Back-to-Nature Island)
- Giant Parrots (Back-to-Nature Island)
- Albert (on Back-to-Nature Island)
- Minerva Campbell (formerly Founder's Island and Hub Island)
- Minerva Bots (probably formerly Founder's Island and Hub Island or likely Cease to function)
- Humans (formerly Founder's Island, B.R. Island, Hub Island and Back-to-Nature Island)
- Alva (probably formerly Back-to-Nature Island)
- Better Reality Players (probably formerly B.R. Island)
- Finn Mertens (formerly on Hub Island)
- Martin Mertens (formerly on Hub Island)
- Dr. Gross (formerly on Hub Island)
- Susan Strong/Kara (formerly on Hub Island)
- Frieda (formerly on Hub Island, later Founder's Island before leaving the Island altogether with Kara/Susan)
- Two Bread Tom (now deceased)
- The Deceased Majority of Humans on Hub Island and Back-to-Nature Island
- Ocean, which surrounds the islands.