It's a gentle, mature sort-of-romance. You might come across Americans saying "taboo subject matter", but the rest of us can take that with a pinch of salt. A 17-year-old girl (Tachibana) is in love with her 45-year-old boss at work ("Tenchou"). He's a clueless divorced dad who apologises to everyone and seems to have Loser Dad so hardwired into his nervous system that I suspect he's no longer even capable of processing romantic concepts. Does he find Tachibana attractive? I don't think so. He's a dad. He also has a filthy smoking habit and so stinks, as is pointed out by Tachibana's airhead co-worker.
Meanwhile Tachibana is a stoic, elegant creature with insanely long legs and a bad tendency to keep everything bottled up. She's the one who makes all the running, obviously. This is odd since her conversational skills are minimal and she tends to repress everything, but there you have it. She's the kind of character who gets you alert to all her micro-reactions and body language to pick up her emotional state. The animators did a superb job there. She's charming.
Oh, and this is also the kind of show where the scenery and weather will be reflecting the characters' feelings and motivations. That's intelligently and beautifully done too. (It's worth mentioning that the character designs are deliberately retro, though. Personally, I loved that.)
Is it a romance? Yes and no. That's what Tachibana wants, but the show's also about promises, old friends and having lost what you really want to do. Tachibana used to be the star of her school's track and field team, but then she tore her Achilles tendon. (Look at those legs. She can run like an antelope.) She loved running, but she's given it up. If she runs now, she'll aggravate her injury. What's more, though, in her mind, she's locked it up in a box marked "the past" and is instead all about her part-time job as a waitress, because that lets her be with Tenchou. Why has she locked it up? Self-defence? Has she stopped caring? If you gave her the choice of turning back the clock, would she say "yes"?
What's more, Tenchou has his equivalent of that. He had something he was passionate about as a university student. There's been a lot more water under the bridge since then and so these days he's just the manager at a family restaurant, but he and Tachibana will be exploring the same issues from different ends. One has regrets, while the other's in danger of making them. It's interesting to see how well their situations dovetail. Both have a friend of the same gender who represents both a hypothetical love interest and their lost link with their former interests. (The romantic angle is so explicit that I'd hardly even call it subtext with Tachibana's friend, Haruka, but it's subtler with Chihiro. He and Tenchou are completely and utterly two blokes hanging out in pubs and picking up their twenty-year-old wrangles about writing. That "old university friends" vibe is so vivid and well evoked that it took me ages to notice that Chihiro has a usually-female name, a tendency to get extremely close to Tenchou and a feminine character design with long, flowing hair. Conversely, Tachibana's given name, Akira, is more usually given to boys.)
Our two heroes will help each other. This is what Tenchou thinks of himself, towards the beginning... "I'm already 45 years old. I don't have dreams or aspirations any more. I'm already empty and middle aged."
There's something classical about this anime. In many ways (e.g. the art style) it's like watching something from the twentieth century. Haruka has the most nostalgically dated character design, I think, which is surprising since the original manga is from 2014-18. The storytelling's also calm, intelligent and a bit slow, in a way that to me seems to be keeping a slight distance from modern anime.
There are lots of things this show isn't. It's not trashy, sleazy or a garbage fire, which will disappoint anyone with certain expectations of a 17-to-45 romance. It's quiet. It also has an ending that answers its questions delicately enough that you might feel it leaves one foot in the air. There will be people out there writing sequel fanfic with more conclusive endings. This is still an excellent show, though.
The 2018 live-action movie's good too.