Summer, 2000. Eleven-year-old Yusuke and his classmates are camping in a woods when suddenly they see a bright light streak over the treetops and into the woods. The boys take off into the woods towards the light. There in the ground, growing, they find a small round metallic object. Just as Yusuke reaches to touch it, the mysterious object up pops a set of eyes and the object says;
"I am Tetra, I meet Yusuke". Startled Yusuke replies "Your name is Tetra? Cool!"
Tetra is kept from adult eyes in Yusuke's closet. Tetra creates wonderful gadgets using "never-seen-before" technology. It would appear Tetra was designed with artificial intelligence. But where did Tetra come from and what is its purpose? As the relationship grows between Yusuke and Tetra, these questions are answered.
Yusuke Sakamoto (坂本裕介 Sakamoto Yūsuke) - Yusuke is the main character. Yusuke hides Tetra in his room.
Tetra (テトラ) - Tetra is a robot who has highly advanced artificial intelligence. Tetra came from the future with the keyword "juvenile".
Soichiro Kanzaki (神崎宗一郎 Kanzaki Sōichirō) - An electronics shop owner who introduces Yusuke to Tetra. Kanzaki is the only adult that the children trust with the secret of Tetra.
Misaki Kinoshita (木下 岬 Kinoshita Misaki) - One of Yusuke's friends. She wishes to connect Tetra to the internet.
Noriko Kinoshita (木下範子 Kinoshita Noriko) - Misaki's sister in law. Noriko becomes one of Soichiro's allies in his Physics research.
Hidetaka Ohno (大野秀隆 Ōno Hidetaka) - Hidetaka is Toshiya's "gopher." Hidetaka, who is thought to be the source of information at his school, is easily able to find information and proves handy to Yusuke and his friends.