This heartwarming sci-fi story follows Kokoro, a girl living in a Martian colony, as she grows through various encounters and adventures. While crimes committed by Knockers are rampant on Earth, Kokoro lives in a Martian colony where Knockers are said to be absent. One day, after visiting her adoptive mother, Masako, who is grieving the loss of a miscarriage, Kokoro encounters a puppy named Koro, who was sent from Earth, on her way home. Later, when Kokoro's life is in danger, she is saved by Koro, who possesses the ability to act as a Knocker…!?