Prologue
Since the tsunami destroyed the nuclear power plant and nuclear waste was poured into the ocean, no one thought the damage to ocean life was so extensive; until an octopus was thrown by the waves into a large concentration of nuclear waste. The waves caught it once more and returned it to the ocean, but by then the damage was done and it began mutating into a horrible monster.
It grew in size until it was bigger than most Japanese buildings and it developed countless more tentacles. The new monster had only one goal: to capture the humans, who it blamed for its grotesque appearance, and punish them by raping them to death with its tentacles.
No one knew how to deal with the threat without risking millions of innocent lives, but they discovered that the creature had one very odd weakness, for a giant octopus, it couldn't open doors or break windows with its tentacles.
Realizing this, the people resolved to stay inside their houses, but this was also a flawed system as they quickly discovered they would eventually run out of food and have to leave their houses sooner or later, and going outside was almost certain death.