One day, a mother Marisa comes out into the forest. She announces that it's a sunny day, and her children follow: A normal Marisa koyukkuri (who is the eldest sister) and afterwards... another Marisa. But this one is special: it has a shell instead of a hat. There's then a flashback to when the shelled Marisa was born; a Patchouli explains that it's a rare breed, and that the shell it has is waterproof. The elder Marisa is ecstatic that her sister is special, and so is the mother.
One day, the two sisters are playing together. The mother Marisa comments that she'll treasure her special shelled child. However, an accident happens when the elder sister, rolling on the floor, collides with the shelled Marisa. The mother is quick to rush and check if Shell (as she is nicknamed) is fine, but ignores the cries of pain coming from the elder sister, who has been injured on the spikes off the shell. She complains that Shell's "hat" can't take it easy, and bites it. The mother comes rushing in, however, and screams at her for attacking Shell, and demands that she apologizes to her. Marisa runs out crying.
Afterwards, there's an explanation that the weight of the shell prevents the Yukkuri that is using it from moving or jumping. This results in the parent having to do everything for it, which results in it being spoilt and the neglect of the rest of the family; the mother would only let Shell ride on her hat during walks; the mother only helps Shell to eat; the mother only takes out and helps Shell when she needs to pee.
Some time later, a Reimu/Marisa family learn about Shell Marisa risa and come to visit. They all play with each other, except with Shell Marisa. A Patchouli asks for everyone to take it easy together, but the elder Marisa runs off, saying she can play by herself. Concerned about her behaviour, and the treatment that she may be getting in light of her shelled sister,Yukkuri Patchouli goes to talk to the mother Marisa. Mother Marisa comments that she is giving the elder sister care, with food and a bed, but Patchouli says that's not the kind of care she means. She's cut off, as the mother storms off when the play between the koyukkuris turns aggressive.
A few days later, the mother Marisa leaves Shell Marisa and the elder sister at home while she looks for food. Shell begins singing, but elder Marisa throws a rock at her and tells her to shut up. She then climbs on top of the "table" in the nest, and eats the food left for them. Shell is distressed, and complains that she's hungry too and demands that Marisa should bring some food to her. Elder Marisa challenges her to do it herself. Shell Marisa slides over to the table, but is unable to climb up it. She cheerfully demands for mother Marisa to pick her up... but mother isn't there. When she realizes, she makes the same demand to her sister- and is ignored. Shell Marisa starts crying, and says that she'll do it herself. She tries to climb up to the small table, but slips off. The fall also dislodges the shell. The elder Marisa comments that she feels bad for not acting right, and hops off the table and lets Shell Marisa use her hat to climb up. There's no food left, and Shell starts crying. Elder Marisa then calls Shell Marisa "scum" for stealing mother and not sharing her. She then challenges Shell to slide off the table by herself. Shell Marisa does so, but because she is so weak, she seriously injures herself, and starts wetting herself. Elder Marisa pee-pees on top of her, and goes to sleep.
However, a short while later, elder Marisa is attacked when the mother returns. The mother screams at the elder sister for attacking her baby, and proceeds to torture her by constantly jumping up and down on her body in order to crush her to death. A flashback occurs, to when the mother was hunting. She's approached by the Reimu from earlier, who tells her that she'll be in big trouble if Shell Marisa is harmed in any way. Afterwards, mother Marisa goes back to the nest, only to find elder Marisa sleeping and Shell half-dissolved and smashed. The flashback is over, and the elder Marisa is finally crushed to death while mother Marisa calls her scum.
Mother Marisa cries over the death of her special child, and asks why it had to happen.