Setsuna Explains Chibi Usa's Strange Illness

Setsuna explains to the others that Chibi Usa's illness is being caused by Nehelenia kidnapping Mamoru. If Mamoru fails to break from Nehelenia's spell, the prince and princess of the future will never marry and therefore Chibi Usa will eventually cease to exist.
Key Cel (A1 End)
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Sailor Stars Episode #170
Destiny disrupted! The sailor senshi in trouble
Unmei no Ichiya! Sailor Senshi no Kunan