Ten-year-old Tsubasa Shiina lost her mother when she was born. Because of this, she has felt a sense of guilt on her life. She is very shy and unsociable to those around her.

One night, she sees a UFO crash in the forest near her home. Seeing that the ranch dog Tenmaru ran into the forest, Tsubasa rushes to fetch him. There, she sees the remains of the downed spacecraft, its semi-conscious pilot and a hideous alien organism called a "Maguar". Running desperately for her life, she goes inside the spacecraft - only to be cornered by the Maguar. But just when her life is about to be taken, a mysterious organism called a "Ribers" fuses with Tsubasa to become the battle unit Figure 17 and defeat the Maguar.

Tsubasa gets an even bigger surprise when the Ribers separates from her and takes the form of an identical copy of her. That copy names herself Hikaru and becomes Tsubasa's new twin sister.

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