Something strange is going on in Tokyo. Despite it being February, it hasn't snowed once this year, and insects are dying off at an alarming rate. While politicians and scientists debate about the effects of global warming, the populace remains completely oblivious to and unprepared for what is coming next.
Chief Togakushi, a widowed engineer who works for a regional power company, is called upon to inspect a newly installed wind farm that is about to go into operation. Along with his lovestruck coworker Asagiri and a guide from the prefecture government, Togakushi notices that none of the equipment on site is receiving power from the basement generators. When the group heads below to investigate, they are stunned by what they find: the basement is teeming with hundreds of thousands of starving rats who have been chewing through the electrical cables. And, as their unfortunate guide soon learns the hard way, the swarm is quick to turn its attention to the new source of food that has just stumbled upon it.
As Togakushi and Asagiri rush to find a way to contact headquarters after discovering that their cell phones are mysteriously not working, the widower's panicked thoughts turn to his high school aged daughter, Kanon, and bug-obsessed younger son, Ryuuto, when he looks into the horizon and realized that there are no lights coming from the direction of Tokyo.
His fears are well founded, as the swarm he encountered is nothing compared to the ravenous plague of rats that is descending on the city. As the Togakushi family rush to find each other before night falls and they are left with no place to hide, scattered emergency radio reports indicate that the entire country of Japan may be under siege.
Tropes present in Region include:
- All Love Is Unrequited: Asagiri really wants to start a relationship with Togakushi, her co-worker. The problem is, Togakushi is totally oblivious to it.
- All Men Are Perverts:
- Any male besides Togakushi, Kizaki and any other main character will be hit with this trope mercilessly. This includes the surviving men that were sent to exterminate the infestation.
- This trope is why Asagiri's grandmother makes her leave home with Togakushi on the trip to Tokyo, as Asagiri's beauty and age would make her a ripe target for her family's more unscrupulous neighbors (though the grandmother had a second reason for doing so).
- All of the Other Reindeer: Ryuuto is constantly bullied by his classmates and homeroom teacher for being a Beetle Maniac and Friend to Bugs.
- Asshole Victim: All over the place, just like in Bio Meat. Most of the time, the victims are Gonk.
- Beauty Equals Goodness: As in the author's previous manga, if you're a Gonk then you're an asshole (who'll likely end up on the menu sooner than later).
- Beetle Maniac: Ryuuto, who is willing to risk bullying from his classmates and elementary school teacher over his refusal to let them kill the insect collection they were keeping as a class project. This leads to him sleeping with the insects in his bed to help them stay warm, much to his unsuspecting sister's chagrin.
- Big Damn Heroes:
- Kizaki saves Ryuuto and Kanon are surrounded by rats in the elementary school by driving into the school's entrance with a mini-van.
- A flock of crows saves Kanon and Ryuuto from certain death while visiting their father's house, picking off and eating the rats.
- Big "NO!": After Togakushi and Asagiri barely escape a flood of rats in the first chapter, Asagiri sees some managed to slip by and freaks out. Later, Kanon utters this trope after seeing the still-living body of an older man latch onto her leg while being eaten by rats.
- Bishōnen: Kizaki.
- Cruel and Unusual Death: Besides being eaten alive by rats, people can also die by losing blood to mosquitoes and being stung to death by ants.
- Dirty Coward: If someone is Gonk, expect them to be this. Best example would be Ryuuto's elementary homeroom Sadist Teacher who left his students to be devoured by rats.
- Dying Moment of Awesome: Kizaki and Hayase running/wading through a very sea of hungry, flesh-eating ants to carry Kanon and Ryuuto to safety, then going back to get them a boat, and only showing minimal signs of pain as they're being eaten. Considering how everyone reacted to being eaten prior, this is quite a feat.
- Eaten Alive: The fate of those caught by the rat swarms.
- Everything Trying to Kill You: Besides rats, the main protagonists also have to deal with ants and mosquitoes.
- Feathered Fiend: Inverted. When it looks like Kanon and Ryuuto, trapped on the roof of their house, appear ready to be devoured by the surrounding swarm, a flock of crows descend from the sky and devour the rats, inadvertently saving their lives in the process.
- Friend to Bugs: Ryuuto believes that all bugs have the right to live as any other creature on Earth. This earns him the scorn of his classmates.
- Gaia's Vengeance: Discussed. It's been suggested frequently that the swarm of animals such as rats and insects were the direct result of humans screwing up the Earth's environment.
- Gonk: Just like in Bio-Meat: Nectar, lots and lots of unsympathetic secondary characters look like they were beaten with the fugly stick nine times over. Even the children aren't safe from this trope.
- Good Smoking, Evil Smoking: Ryuuto's homeroom teacher, aside from being Gonk and being responsible for the boy being bullied by his classmates, is a constant smoker, making him a very unpleasant person all around.
- Green Aesop: Again like in Bio Meat, the source of the problems (the massive insect die-off and the rat population explosion, who no longer have insects as a viable food source) is humans screwing up the environment. Unlike the previous manga, however, this is something you could almost imagine happening in real life, seriously amping up the creep factor.
- Half the Man He Used to Be: When Kanon's ankle is grabbed by a man who is pleading for help, she is horrified that everything below the man's waist is covered with rats, happily gorging themselves on what little remains of his flesh.
- Heroic Sacrifice: Kizaki and Hayase carry Kanon and Ryuuto to the ocean to save them from ants, then go back to grab a small boat for them to use, knowing that the ants will overcome and devour them.
- Humans Are Bastards: Used interchangeable with the manga's Green Aesop. Best example comes when one of the group of survivors huddling in the elementary school for refuge beats a pest control worker with a chair in a rage and panic when he reminds the other survivors that the invading rats will double their numbers until they overrun the area and endanger the human population. The rest of the survivors (barring the heroes) cheer the attacker on. Kizaki lampshades the trope in the beginning, noting how the much world would be a much better place if people weren't busy being assholes.
- Hypocritical Humor: In the first chapter, Kanon criticizes Ryuuto's habit of letting bugs sleep with him on his bed. Tagokushi points out that Kanon once did the same thing with a hamster she found. Kanon argues that hamsters are cute, unlike bugs.
- Irony: There are a couple of Gonk girls in Ryuuto's class who fret over how a school lockdown wouldn't be good for "beauties".
- Kids Are Cruel Ryuuto's classmates, in addition to being ridiculously Gonk, Kick the Dog when they crush one of his cherished Hercules beetles. Why? Because they got annoyed when Ryuuto proclaimed their right to live and responsibility to their care against them wanting to get rid of them because they thought they were useless.
- Missing Mom: Ryuuto and Kanon's mother died before the events of the story.
- Nice Guy: Ryuuto doesn't have a single mean bone in him. When the school is infested with rats, his first thought is to go try save his classmates, the same classmates who relentlessly bullied him. Kizaki notes how the world would be a better place if people thought just like Ryuuto.
- Nothing Personal: Ryuuto's Sadist Teacher thinks this trope as he abandons his remaining students (after two are eaten and Ryuuto leaves) to the rats.
- Oh, Crap!: Ryuuto's homeroom teacher gets this when he saw that the boy survived the rats thanks to Kizaki and worries he'll be exposed for throwing Ryuuto's classmates (who all weren't very nice to begin with) to the rats.
- Papa Wolf: Togakushi, who will do anything to make sure his kids are safe.
- Parent with New Paramour: While Chief Togakushi seems oblivious to the feelings of Asagiri, Kanon is quick to notice the way she acts around him. This drives a wedge between the father and daughter, as she feels that it's too soon for him to abandon the memory of her mother. It doesn't help that, when her hair is up, Asagiri looks like Togakushi's dead wife.
- Reasonable Authority Figure: Kizaki, an elementary school teacher who's quite mature and helpful. When he and Ryuuto find two school children being eaten by rats in the restroom, Kizaki covers Ryuuto's eyes so he won't see the carnage.
- Sadist Teacher: There's one teacher in Ryuuto's class who's behind the boy's constant bullying. During a school lockdown caused by a power outage, he decides to blame the situation on Ryuuto and ponder about whether he should increase his bullying or rape him. He also abandoned his students to die after the school is infested with rats so he wouldn't be held responsible for their safety.
- Spiritual Successor: To Bio Meat. Both share the same story formula (Japan is afflicted by a country-wide apocalypse due to rampaging critters), almost every character is Gonk (except for the main characters save for one who was a protagonist), and both share a Green Aesop. Some of the characters here are similar to Bio Meat as well in both looks and personality.
- The Stoic: Kizaki, whose face doesn't emote beyond a half-lidded expression.
- Swarm of Rats: Now with ant and mosquito swarms. Also, roaches.
- Tampering with Food and Drink: Ryuuto's Sadist Teacher tries feeding him a poisoned rice ball to avoid the chance of being outed for his actions. Luckily Kizaki saves Ryuuto just in time.
- Used to Be a Sweet Kid: Kanon used to be a lot cheerful as a kid and more close to her family before her mother died.