Plot
A large robotic craft arrives searching for Susan Strong (voiced by Jackie Buscarino), but the craft is dispatched by Jake (voiced by John DiMaggio). Princess Bubblegum (voiced by Hynden Walch) examines the wreckage and discovers that it is of human origin, which engenders in Finn the Human (voiced by Jeremy Shada) a desire to travel across the sea from whence the craft came and meet others of his kind; Susan, Jake, and BMO accompany Finn on his voyage. Initially, the journey is easy and uneventful, but after a series of mishaps, the group's boat is destroyed by a mysterious colossus, throwing everyone overboard. Finn subsequently wakes up on an island where the weather drastically fluctuates. After meandering around for a while, he eventually encounters an old lady named Alva (voiced by Helena Mattsson) who speaks Swedish. Alva invites Finn to her home and shows her home movies of other humans who have since presumably died. Later, Finn and Alva encounter Jake, who informs the group that he does not know the whereabouts of either BMO and Susan. Finn and Jake eventually head to a futuristic island where all of society has hooked themselves up to a virtual reality simulator. BMO is revealed to have become the heroic leader of the VR people. Jake decides to go and destroy the generator powering the VR after BMO refuses to leave. Feeling bad for BMO, as well as the emaciated humans who emerge from the virtual reality simulator, Finn asks Jake to fix the generator, but BMO fixes it himself. BMO recognizes that if he remains behind, he will lose his friends, and so he, Finn, and Jake take a pod to the next island. It is on this stop that Finn, Jake and BMO find Susan, who begins to recall her long-forgotten past.
An extended flashback reveals Finn's origin, and what happened to the other humans. Roughly a thousand years prior to the main events of the series, a group of humans fled Ooo on a container ship (as seen in Stakes). They eventually settled on a secluded island chain far from the mainland. Over the next thousand years, the community grew into a veritable utopia. With that said, there were those few who occasionally grew dissatisfied with their rigidly structured lives and attempted to flee the island; these "hiders" were in turn hunted down and returned by specially trained "seekers". It is revealed that Susan's real name is Kara, and that she was once a seeker in training. Kara was friends with a fellow human, Frieda (voiced by Jasika Nicole), who began expressing a desire to flee the island. This revelation causes Kara some discomfort, so she approaches Dr. Gross (voiced by Lennon Parham), the cybernetic human in charge of training seekers, asking if the humans can live off the island. Dr. Gross convinces her that the outside is dangerous and, to prevent her from fleeing, she controls Kara with a remote control. Now fully under Dr. Gross's control, Kara stops Frieda from leaving and drags her away crying. Back in the present, Susan tells Finn her real name and decides to take Finn to Founders Island so that he can be reunited with his mother, Minerva Campbell (voiced by Sharon Horgan).
The audience is then presented with a series of flashbacks detailing how Minerva, a doctor, met Finn's father Martin Mertens (voiced by Stephen Root) when he was hospitalized after it was mistakenly believed he was attempting to leave the island with a group of escapees. Martin and Minerva eventually fell in love and had Finn. However, when the group of attempted escapees sought revenge on Martin for a previous betrayal, he fled on a boat with Finn. His escape is thwarted by the colossus—revealed to be a security device that was created to protect Founders Island from outside threats—which attacks the craft. In the chaos, the pair are separated, leaving Finn to drift away. The group is arrested and Minerva is heartbroken over Martin and Finn's disappearance.
Back in the present, the group make it to Founders Island where humans have been living in relative peace. While Kara seeks to make amends with Frieda, Finn and Jake discover that Minerva had digitized her consciousness in the past and now exists only in virtual reality. Minerva reveals that she had Dr. Gross send Kara to retrieve Finn, but years had passed and Dr. Gross had accidentally released a deadly virus that was killing humans. Minerva had her essence uploaded into a computer, and then created the helpers to assist the human race. Now that she is with Finn, she expresses her desire that he stay permanently. Finn tries to convince Minerva that life off the island is not all bad, but Minerva thinks off-island life is dangerous. Finn then tries to convince the humans to leave, and they all rally alongside him. This causes Minerva to attempt to upload the consciousnesses of all the islands' inhabitants. To prevent this, Finn shares with her his memories of helping people, causing her to back down; she realizes that Ooo is not nearly a threat as she before believed. The humans all change their minds about leaving, except for Frieda, who announces to Finn, Jake, and BMO that she and Susan have made amends and are leaving to have their own adventures. While setting sail on a new boat bound for Ooo, Finn has one final talk with Minerva through the VR headset, where the two embrace in the digital realm.