Subaru is a young girl who likes to see the stars. One day, she opens the door to her school's observation room, only to find a large indoor garden instead! She meets a boy named Minato who says some strange things, telling her to leave soon after their meeting. As if the day wasn't weird enough, Subaru accidentally finds a strange blobby creature who runs off with her compass, leading her to stumble into a club room with girls wearing witch costumes! Subaru recognizes her friend Aoi in the group, and despite Aoi's protests, Subaru decides to join the club. Shortly after, she gets a strange automobile-like staff and a magical transformation from the club's "president," the blobby creature from earlier, known as a Pleiadian. Aoi and the other members—Itsuki, Hikaru, and Nanako—have been looking for engine fragments of the spaceship that the Pleiadian used to travel in, so that it can go back to its home. But it seems that these girls are not the only ones searching for the fragments...

The sky is the limit in Houkago no Pleiades. With telescope in hand, Subaru is set to go to the observation room of her school in order to get a view of that night's meteor shower. What she least expects is that behind the observatory door was not the starry skies, but a lavish garden, complete with a resplendent fountain and a mysterious young boy with long red hair. But the garden soon disappeared, as if Subaru was only imagining things. All that remains of that brilliant sight is an odd, bouncing blob creature that leads her to another magical door, occupied by other girls in magical witch-like costumes. Revelations start hitting Subaru one after the other: one of the girls in the room is her childhood friend Aoi, the little blob is actually an alien of a species called the Pleiadians trying to return home, and Subaru has been selected by him to become the newest member of their group! Now Subaru's dreams of the stars have come true in the wildest way, as she and her friends attempt to gather pieces of the Pleiadian spacecraft engine to return the being to his home. But they're not the only ones after the engine parts, and they have no idea why!