it's about a gachapon machine with girls in it... you put a coin in, turn the knob, a capsule with a girl drops out. You open it, add water - instant girlfriend
...
But yaknow how some gachapon machines have a special ball in them? Our protagonists got the opposite of that and got the cursed black ball, and his classmates are haranguing him to open it and add water
"Salt cures Everything!"
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