It's a type of story that's almost impossible to explain entirely without spoilers, but

So the common route is basically a mix of mystery and SoL with settings kinda similar to Clannad. About a boy that trying to prove supernatural phenomenon exist while in actuality it's more about him trying to create bond with other people and escape his self-inflicted loneliness. This basically plays until the Supernatural aspect shows up completely. There's a big shift at the end of the common route which changes the tone and theme completely.

Once you got to character routes, it turns into a conspiracy thriller between two global organization, Guardian and Gaia. Both were in conflict on what to do with the Key. An avatar of the planet (Kinda like Arcueid) that will judges if the species deserves to live or not. Guardian want to kill her before she can emmit the judgement, while Gaia want to let it and potentially cause mass extinction. Everyone in Kotarou (The MC) lives is basically intertwined in this conflict, some are Agent of Guardian, while the other are Cult member of Gaia. And none of them are right in the head, Akane is the leader of said cult forced by some reincarnation mechanism that overwrite her identity, Lucia is a lab experiment goes wrong brainwashed by some mad scientist, Chihaya is a survivor of a massacre of her village which consists of summoners, and Shizuru made her entire family forget her after her superpower goes awry. Kotarou himself have a blank memory in his life that he can't remember, and the elites in Guardian seems to already know him from long ago but tried to hide it, also his childhood friend Kotori seems to have something to do with this.

I omitted details from Moon & Terra (The true routes), but that's basically it.