Hiroshi, Satoru, Susumu, Takeo, and Yasuko are five unique children who have different talents, such as being skilled at computers, being an expert at throwing stars, and excelling in studies and sports. The leader of this group, Hiroshi, creates a fictional character, the phantom Matenrou on his computer. When the five children shoot a video about this phantom and play it in their school, a small problem emerges. That evening, when Hiroshi finds that his precious computer is gone, Matenro, who is real appears before him and his friends said “If you want your computer back, accept my challenge,” Accepting the challenge, the children decides to form their own detective squad, “Hard Gumi" to catch Matenro.