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 remaining guest realizes the sister's motive is to avenge her brother's death.
3) As the last guest escapes, the sister's drowned brother emerges from the lake and drags her under to drown as well, ending her killing spree.