Departure's main plot is divided into two parts:
Part I begins after the events of a "camping trip." Forrest was supposed to invite his friends Junko and Ximena on a standard camping trip, but after an encounter with a bizarre ghost girl in his grandmother’s home, they end up camping in the Afterworld instead. There they run into Mortimer, a young prince who snuck out of the palace to go on a walk. Bored and disillusioned with his life, he ends up joining them back to the world of the living.
This part spans almost three years, a lighthearted slice-of-life style of narrative about the strange situations they get into because of their new friend.
Part II begins three years later, when Mortimer is finally found by a search party sent by the royal family and forcibly sent back to the Afterworld and his home country of Solneshny. Back home, he is under almost constant surveillance and cannot leave his bedroom unaccompanied. He is also forced into an arranged marriage and is more or less obligated to spend time with his fiancée.
Not quite a lighthearted slice-of-life anymore, the story leans more into the drama and conflict that comes as a result.
The Afterworld, simply put, is the land of the dead. It is a physical place that can be visited by the living as well as the dead. Large bodies of water such as lakes, oceans, and rivers serve as a “portal” to and from the Afterworld and the living world. The only ones able to intentionally travel between both worlds are psychic mediums and ghosts themselves. If unassisted by a medium or a ghost, any non-medium living people end up there by accident. This is the very reason why living human civilization exists in the Afterworld. Specifically, accidents like shipwrecks or fleets of ships going unexpectedly missing. Such events cause the people to settle in the Afterworld, and although it has different geography than the living world, it is perfectly habitable.
This is the origin of the nation of Solneshny, where Mortimer is from. Solneshny, being established in the mid 1600s, has its own long and detailed history, as do all of the settlements in the Afterworld.
Though very few people in the living world know of its existence, the opposite is true for the people living in the Afterworld. The only people from the living world who know about the existence of the Afterworld are the original Departure three (Ximena, Forrest and Junko), psychic mediums, and the odd person with knowledge on it for whatever reason. Perhaps intelligence agencies do too? Meanwhile, the vast majority of humans living in the Afterworld are aware of their history and know how their respective nations came to be. They are also all believers in ghosts, unlike in the living world where to most people ghosts are but a superstition.