Miyu Kouzuki is an 8th-grade student whose parents leave for the United States to work for NASA and arrange for her to stay with their long-time family friend, Hōsho Saionji, a monk who lives in an old temple on a hill overlooking the town of Heiomachi. However, Hōshō leaves soon after on a year-long pilgrimage to India, leaving Miyu to stay in the same house with his son, Kanata.

Suddenly, a UFO lands in the honden of the temple. Inside is a humanoid alien baby, Ruu, and his cat-like "sitter pet", Wannyā. They arrived on Earth from their home planet, Otto, when they fall into an interplanetary wormhole (a time warp which can transport objects across several regions, planets and time planes). They cannot return to their home planet because it is too far from Earth; Wannyā asks Kanata and Miyu to allow them to stay in their house, and they agree. Miyu and Kanata learn from Wannyā that people from Planet Otto look identical to human beings, and that Wannyā can also transform into human beings, animals and objects.