Okay, the trailer for this lied really hard.
It looked like a harem show, with a bit of light comedy and some unnecessarily soppy girls who are all obsessed with the protagonist, mostly because he's the protagonist. That is massively not true.
It starts as the trailer suggests, with Sakuta (our main character) meeting Mai, a famed actress, when he's in the library one day. He's more than a bit confused in that she's wandering around dressed as a bunny girl, and yet no one seems to notice her. (And trust me, as a fond user of libraries, I can say a bunny girl would stand out.)
She's very surprised that he sees her, because increasingly, people don't. It seems like an odd supernatural effect is spreading throughout their school, affecting those going through puberty, and its effect on her is to wipe her out of people's memories.
As time goes on, more and more people are forgetting she exists, and are able to even see her at all. Sakuta believes her (many don't) because he's gone through this weird supernatural problem before. His sister suffered actual physical injuries as a result of cyberbullying, and at one point he was even hospitalised (though has no memory as to why).
But that loss of recognition is painful. Mai's existence is becoming lonelier all the time, and her budding romance with Sakuta is the only thing keeping her sane. This gets impressively bleak, with her depression hitting hard and his determination to never forget her being unexpectedly deep. This show is a lot more emotional than you will ever expect.
A significant part of the fun of that is their relationship. They're both very dry, sarcastic people, and the quiet romance they build feels very real. She's effectively tied to him without any real choice, but he has to choose to stick with her. The writing is good enough that you can believe that he would. And it wonderfully avoids the problem that so many teen shows do where they're all overwrought with horniness.
That's the arc of the first three episodes, and honestly, it could stand alone as a feature film. It reminds me a lot of Your Name, and I admit to being both excited and worried about what they're going to do for the other nine episodes. It looks in a certain manner to be akin to an anthology show, so I'm expecting four arcs of three episodes each, each of which deals with a new and troubling problem.
To be honest, though, this first arc is good enough that I can heartily recommend you watch it. Even if the rest of it falls off a cliff, that arc is actually powerful. This is going to be on my Autumn "Best of" anime list.