A mysterious craft invades Ooo and Finn believes it may hold the secrets to his past. 
A human transport ship lands at a beach and scares a Candy Person, prompting Mr. Cupcake to investigate. The transport ship identifies Mr. Cupcake as hostile and burrows him into the sand, then flies ahead towards the Candy Kingdom. Finn, Jake and BMO attack the ship after it easily defeats the Banana Guards and Gumball Guardians. Before Finn could attack it, the ship recognizes him and addresses him by his last name, Mertens. Finn is shocked when the ship invites him inside and asks for help on locating "X-J-7-7 Strong" but is appalled when Jake suddenly destroys the ship. BMO kicks the wreck and accidentally triggers a mechanism that connects to it, displaying the ship's designated target: Susan Strong.
At the infirmary, Susan awakens to Finn apologizing for his grass self attacking her to which Susan apologizes for attacking them. Outside, they join Princess Bubblegum and Jake as they investigate the wreck which, along with Susan's head implant, are confirmed to be of human origin. As Susan sits in the wreck's cockpit, she suddenly regains parts of her memories and reflects. Bubblegum has BMO print the ship's final commands: locate "X-J-7-7 Strong" and return cargo to Founders' Island. Susan decides to journey to the island but Bubblegum objects to the decision due to the island being too far away. Finn subsequently decides to go to the island with intentions of finding more about his past and the rest of humanity, with Jake agreeing to come. Princess Bubblegum offers her boat to them as transport.
At the tree fort, Finn tells Fern to take on his role while he is away but Fern tells him that he can't fill in such a role. Finn consoles him by telling him to "just do it your style" and leaves to pack. Jake says goodbye to all of his puppies, then to Lady Rainicorn. They meet at the tree fort, to which the two say their goodbyes to. Neptr waves goodbye and is dumbfounded when BMO isn't around.
At the beach, as Susan is loading all their stuff into Princess Bubblegum's boat, Princess Bubblegum and Marceline say their goodbyes to Finn. Bubblegum reminds Finn to be careful due to what he might encounter be heavy for him to which she makes him promise to her that he come home safely. Fern jumps in and introduces himself to Bubblegum but admits to having half memories of her due to him originating from Finn. Bubblegum and Finn share a hug as Susan says they are ready.
Fern, Bubblegum and Marceline wave goodbye to the trio. Having left the Land of Ooo, Finn takes off his hat and looks at the map printout of the journey, anxious to reach the end of the journey.
- Human Transport Ship
- Lady Rainicorn
- Frieda (hallucination, cameo)
- Candy People
- Charlie seems to have moved to an apartment and is no longer living in a pyramid like she was in "Daddy-Daughter Card Wars."
- The outfits that Princess Bubblegum and Marceline wear seem to be inspired from the outfits Natasha Negovanlis and Elise Bauman wear in the webseries Carmilla. Princess Bubblegum's outfit resembles the outfit Elise wears in the episode "Warning Signs." Marceline's outfit has similarities to Natasha's outfit from Season 0.
- Susan seems to be recovered from her injuries she got in "Reboot" and is in far better condition she appeared to be in during "Do No Harm."
- The ship was called by Susan's implant in "Reboot".
- Marceline mentioned Fern totaling Starchy's motorcycle. Fern was seen taking off with it in "Do No Harm."
- This is a continuation of the events of "Reboot", and the first episode in the miniseries "Islands".
- This episode was released for digital download and DVD on January 24, 2017.