OverIsland is set in a mythical place called Tidulidde (Tide-ool-ide), a big island situated in the ocean that consists of waterfalls, farm land, snowy mountains, villages and Savannah. A route can be taken from Waterfall to get there. About fifty or so skeleton-monster-like creatures live there, each having the unique ability to shape shift into an animal of some sort. Each inhabitant is sorted into one or two ranks. Some of them are born with these ranks or have to achieve them. The ranks are: Helper, Fighter, Secret and Unclassified (apprentices). Their languages consist of mostly English, Wingdings and Tidulian (their native language). The island's main language is English and their national animal is a dog. Humans sometimes settle on this island when they get lost. Everyone on the island is friendly and will only fight if they feel threatened. Eventually as the game progresses the island is destroyed and set on fire but some of the islanders escape and travel by boat to reach the Underground. The citizens are easily mistook for monsters.
You play as Eras and go through the Underground and visit the Surface. You learn more about your past and meet Lionel at the end.
The game starts at the scene where Flowey ran away in Undertale. You play as Flowey stranded on an Island called Tidulidde (Tide-ool-ide), a massive island with strange skeleton-like creatures that connect with their animal
ancestors. You stay for a few nights. To unlock this route you must find Florence.
You play as Lionel on Tidulidde. At the end you wake up in this strange 'purgatory' place and meet your long lost
sibling. To unlock this route you must kill Moon.
You play as Eras and meet Eric at the end who has became an amalgamate. If you spare him the game lasts longer but if you kill him Error Sans will appear. Florence will follow you and try and help you stay alive. To unlock this route you must help Travis find his dog.
You play as Eric and travel with Florence. Later you can choose to play as Robyn or Jackson. To unlock this route
you must help the islanders on Tidulidde during the war.