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A mysterious treehouse hidden from adults is the headquarters for five friends known as Kids Next Door. These 10-year-olds take on adults to get out of going to the dentist or summer camp by using "2x4 Technology.

The year is AD 2225. Kouji Aiba and Aoi Housen are serving as astronauts in-training in Liebe Delta which is located on the edge of the Geduld Sea.

The story centers around five Japanese high school students who are too young to be called adults, but who no longer think of themselves as children. Wakana Sakai once took music lessons, but she withdrew from music after losing her mother.