Hikaru Takabe may not be the most social of teens. Always sporting her headphones, she gives off an aloof aura that rubs her classmates the wrong way. But her not being part of the crowd takes on a different dimension when she becomes involved in an intergalactic game of cat and mouse.
Inspired by Needle, the Golden Age hard sci-fi classic by the late Hal Clement, Nobuaki Tadano's debut work brings a unique take on alien invasion up to date and into the maelstrom that is the Japanese high school girl.
- Alien Invasion: The alien Maelstrom takes a host and then goes on a killing spree. Horizon follows him an attempt to stop him. Repeat ad infinitum.
- Apocalypse How: According to Horizon, Maelstrom is trying to bring about a Universal Class 4 (and it's implied that he already did this to Earth once, per the extinction of the dinosaurs). Later on, the Moderator plans to initiate a Planetary Class 4, by pushing the Reset Button on evolution and returning all life on Earth to primordial ooze.
- Body Horror: Oh, yes. Expect things to get messy whenever Maelstrom is around. And it gets even worse later.
- But Now I Must Go: Horizon and Maelstrom leave the Earth in the end, albeit having come to terms with each other.
- Gorn: Again, yes. To put it into perspective, we see Hikaru (the main character) get vaporized layer-by-layer at the beginning of the very first chapter. And that's just the beginning...
- Defeat Means Friendship: Maelstrom, surprisingly. And again, later on with Chika.
- HeelFace Turn: Maelstrom.
- Headphones Equal Isolation: Hikaru. She abandons them after her Character Development. In the epilogue, she runs into a boy who seems to embody this as well. He even appears to be "talking to himself" in the same way that Hikaru did with Horizon.
- Humanity Is Infectious: At the start of the manga, Horizon is generally a nice guy but doesn't quite "get" a lot of human quirks or the reason why Hikaru does she does sometimes. Over the course of the story he starts to learn more about being human and thus becomes more "human" himself. This is also the case for Maelstrom after he gets trapped inside Hikaru
- Humanoid Abomination: Hikaru herself is this after Horizon enters her body, though it becomes a lot more blatant after Maelstrom follows suit. After the Moderator starts the macroevolutio, many people become these, notably Chika and Hikaru's aunt & uncle.
- I Just Want to Have Friends: Chika wants to find companions who won't judge her so harshly.
- Kids Are Cruel: When Shunsuke's class has to dissect frogs, the other students in his group know good and well that he shouldn't be around sharp objects, but they do nothing about it. And then he suffers a nervous breakdown that allows Maelstrom to take over his body. Nice Job Breaking It, Students.
- Lovecraftian Superpower: Hikaru's symbiosis with Horizon borders on this, but the trope really comes into play when Horizon manages to take Maelstrom prisoner... inside of Hikaru.
- Meat Moss: After the Moderator triggers the macroevolution this starts to crop up, as a sort of mixture of soft tissue and primordial goop.
- Nice Job Breaking It, Hero!: Horizon manages to find a way to permanently end Maelstrom's extinction spree by containing him within himself (and, by extension, Hikaru). Unfortunately, Horizon and Maelstrom's little cat and mouse game existed to keep the evolution and mutation of life in check, and now that they've both "abandoned their positions", the Moderator has to step in to bring about an Apocalypse How instead.
- Ordinary High-School Student: Hikaru, until she ended up sharing her body with a super-powered alien.
- The Power of Friendship / The Power of Love: Or, as Horizon calls it, the "heart bonds of lifeforms" are what save Hikaru from Maelstrom's nightmare world.
- Secret Keeper: Saya, though she doesn't know exactly what's up with Hikaru, knows that she's involved with some sort of craziness. Nao gets in on some of it later on.
- Shock and Awe: What Horizon's main offensive powers manifest as through Hikaru.
- Sugar-and-Ice Personality: Hikaru.
- Super Speed: Horizon can do this while controlling Hikaru's body.
- Tearful Smile: Hikaru, near the end of the second book. It is beautiful.
- Took a Level in Kindness: While she never entirely ditches her snark, Hikaru becomes gradually nicer and more altruistic as the manga goes on. This also applies to Maelstrom who becomes far less creepy and villainous after being trapped in Hikaru's body.
- Vicious Cycle: Maelstrom attempts to wipe out all life on a planet, Horizon chases him and attempts to stop him. They move onto another planet, Maelstrom wipes out all life, Horizon chases him, they move onto another planet...
- Wake Up, Go to School, Save the World: This is basically Hikaru's life after Horizon shows up. The "Save the World" part goes a little slowly until Maelstrom finally makes himself known.
- You Are Not Alone: Said to Hikaru several times. It slowly becomes true of her as the story progresses and she opens up to other people. While outright not said, she pulls a similar ploy on Chika in the climax.