The setting is a world similar to modern-day Earth, specifically Tokyo, Japan. It was recently revealed that various extraterrestrials exist, and this fact is gradually becoming common knowledge.

These aliens have formed the "Interstellar Union," a coalition of species with advanced civilizations and high moral standards, and they are currently evaluating whether humans are fit to join. Generally, the aliens visiting Earth are more intelligent, mentally stable, and friendlier than humans.

While the members of the Interstellar Union are friendly toward Earth, they lack common sense regarding human society and are still in the learning phase.

A certain amorphous swarm-type alien belonging to the Union decides to send split bodies to Earth for investigation and friendship. These are the three: Ei, Bee, and Cee. Originally one collective entity, they have been given distinct appearances, personalities, and temperaments to facilitate communication with humans. Ei, Bee, and Cee share all memories and consciousness with the main body.

Upon landing on Earth, they happened to encounter Guest and took a liking to them for some reason, moving into Guest's life under the guise of an investigation.

The Union's activities on Earth are officially supported by the government; any minor troubles are handled and followed up by government agencies. Other alien investigators are also dispatched.

Guest's setting is free to be defined.