It's brilliant. Okay, maybe that's overdoing it, but I really enjoyed it and my only gripe is that it's not longer. (It's adapted from a web manga and I wouldn't say that it ends. Instead it finds a good place to stop.)
Apparently the story will be concluded in a four-part 2018 OVA and there's also a 2017 live-action film. I'm definitely watching both of those. Anyway... let's describe the plot!
Kaizaki Arata appears to be a loser. He's a 27-year-old man who hasn't had a full-time job since quitting his first one after only three months. He currently works part-time behind the till in a convenience store. He's also got emotional issues. One day, though, he gets a unique job offer while drunk in a back alley, late at night. (Does this sound dodgy to you? It'll seem even more so when you see the suspiciously smiley man who makes this offer.)
Kaizaki's mission is to return to high school for a year. (They'll give him a drug that makes him look seventeen.) He'll be pretending to be a teenager. He'll be sitting exams in subjects he hasn't studied in a decade. He'll be saying "yes, sir" to teachers younger than him. He'll be extremely careful with his relationships with female classmates, even though his ReLIFE minders say there wouldn't be anything wrong with him finding a girlfriend. (The age of consent varies in Japan, but this doesn't seem to be one of the prefectures where it's eighteen. This has apparently put off some potential American viewers, though.)
Anyway, after the year's over, everyone but Kaizaki will get amnesia pills and forget about him. However the ReLIFE corporation will pay his living costs for the year, give him a minder to make everything go smoothly and guarantee him a job afterwards if everything goes okay. That sounds pretty good, actually. It's a life-changing opportunity for someone in desperate need of one. If I were in Kaizaki's position, I'd accept... and indeed he does. Welcome to the best days of your life, again!
The ensuing series is both great and funny.
Firstly, the generation gap adds more than you'd think. It's not just a gimmick. Instead of having a cast of teenagers like most high school anime, we're getting the extra perspective of adults who can remember their own mistakes. There's more self-awareness than usual. They can compare these 17-year-old idiots with the idiots they used to be themselves. This could have seemed preachy, but not from someone as broken as Kaizaki (although he has his reasons).
Secondly, these teens have issues that are becoming ever more of a problem in real life, not just in low birth rate Japan. They're bad at communicating. Gifted people get cold-shouldered by people they'd thought were friends and develop complexes about their own gifts. Chizuru is the most extreme example and surely has some kind of syndrome. Ouga is a milder case, since at least he has emotions, but he's nearly as dense. Finally there's Kariu, who's just bloody hard work. Having her as a friend would be exhausting. (For example, she's been beating herself up about being only the second-best in her class, both athletically and academically. Well, gosh darn. That hardly sounds like failure to me.)
The themes are interesting. The series explores good intentions and how they can go wrong. How do you tell the difference between good and bad well-meaning intervention? What about the case where the person you're trying to help is telling you to stop it, but the correct action is to keep pushing anyway?
There's also the question of whether it's rational to be rational. Everyone's going to forget Kaizaki. The kids might all end up going to different universities anyway. Therefore, if you take the long view, theoretically there's no point in doing something now that'll just make it harder to say goodbye later.
All those are reasons why the show's interesting, but they're not the reason why it's lovable. It's the cast. No one's a joke character and it's adorable to watch them bouncing off each other and struggling to grow up. (Kaizaki included.) It's a happy, funny show. Dark things have happened, but I always enjoyed spending time with this cast and I really wanted to see them make a success of themselves. One of my favourite anime of 2016.