1) A boy drowns at a lake after meeting some older kids on Bonfire Night. Years later, his sister throws a party for those same kids at the same lake, seeking revenge for blaming them for his death.
2) At the party, the sister begins killing the guests one by one. The last guest realizes the sister's motive and tries to escape.
3) While being chased by the sister, the ghosts of the drowned boy appears and drags the sister under water, killing her as well.