Gon Freecss only has thirty days left to live.
He has contracted a rare disease known as Pharadoxia, and there is no cure.
Even Leorio, the doctor, doesn't know how to help him.
Killua is completely devastated, of course. His best friend is going to die, and he can't do anything to save him.
So he is determined to make Gon's last days his best ones.