It's a romantic comedy about death. The main girl (Isla) is an android who's going to fall in love, then die. (The latter is pre-ordained, although not the former. She has an expiration date. If she hasn't been recalled by then, her personality and memories will decay as she becomes a mindless danger to everyone around her.)
We know all this, almost from the start. She's going to die and we're even told how long she has. Then, in the last episode, she dies.
However it's not a depressing story. On the contrary, it manages to be uplifting and positive. It's about accepting the inevitability of bereavement and not letting fear and distress stop you from living your life.
In ep.1, Isla's terrified. When we first meet her, she's crying in a lift. She has under three months left of her original 81,920 hours (about nine years and four months). Ironically the company that built her is employing her in Terminal Services, i.e. recalling other androids before their expiration dates, but she sees this as "ripping apart memories". She thinks experiencing happiness will only mean more pain in the end when you lose those memories, so it's better to wall yourself off emotionally and only interact with others when it's unavoidable.
Compare this with Isla at the end. She'd been so scared, but now she's calm. She's at ease. She's found acceptance, as to a lesser extent has Tsukasa Mizugaki. It's an interesting final episode, plus of course beautiful and the entire point of the show. I like that it's avoiding the things one expects from tearjerking death scenes in general. It's about understanding. It's about Isla realising that she's been helping people and that it was okay for her to be happy.
"I thought I didn't deserve gratitude. I was mystified when people said 'thank you'."
This is a deceptively simple story. It disappointed some viewers who'd got into the SF side of things, expecting the show to do more to explore the nature of these androids and their effect on society. Is that 81,920-hour limit just built-in obsolescence to force customers to buy regular upgrades? (I think it must be, yes, which is horrible but not explored. We do however learn that our heroes work for a unique branch of Terminal Services, in that they actually try to engage with their clients emotionally. It's more normal just to barge in and exterminate.) What about the black market dealers from eps.4-5? They're disturbing, but then the show moves on and forgets about them. What about R.Security? Yup, them too.
Mind you, there are some disturbing details about what happens to a reclaimed android. You can get the old body implanted with a new personality, for instance. Your friend of 9+ years will have been erased, but you can buy a replacement!
The all-important thing, though, is Isla and Tsukasa. The show's built entirely around them. The setting is consistent and satisfying, but unimportant. The focus and pacing might thus seem odd, with Isla and Tsukasa getting together so late in the day that you might be left wondering how the show's going to cover everything in the last couple of episodes. (That's what I was thinking, anyway, but that was a misunderstanding based on flawed working assumptions of what the story was going to be doing.)
Similarly the tone can seem eccentric. It's quite light and fluffy, despite the obvious emotional weight of mortality, despair and what are effectively android executions. There's a lot of comedy, some of it too silly to be funny. (Some of it, mind you, is too harsh to be anything but a painful insight into someone who's deteriorating. Isla is clumsy. In ep.1 this is funny. In ep.2 it becomes uncomfortable, because this is a dying person who's slowly losing motor functions.)
Nonetheless we have a comedy workplace, where everyone's implausibly cheerleading their colleagues' office romance. We have Tsukasa being almost as big a doofus as Isla, for instance with his embarrassing antics to try to start a conversation in ep.3. (They're perfectly suited to each other. They're both dorks.) This is a happy series, not a depressing one, and it achieves this with lightness and smiles. Do they get the balance right? That's up to you. You might think "no" if you're trying to second-guess what you think the story's about, e.g. you're expecting a Big Twist (ahahaha, no), or else you think the show should have been darker and more focused on its SF elements. Personally I'd defend it to the death, but at the time even I sometimes found it a bit silly. Ep.9 is out-and-out romantic comedy, for instance, perfectly goofy and feather-headed until we learn about Isla's unhealthy worldview.
Their colleagues are a laugh. They have an intimidating hard-drinking boss (Kazuki), but she's not actually the real boss and no one pays much attention to the bald middle-aged guy who is. (Apparently his daughter doesn't either.) There's a woman-chaser (Yasutaka) who talks garbage and you'll want to throttle Tsukasa every time he seems to be listening to the guy. There's a tsundere redhead (Michiru) and of course everyone's android partners, all more mature and level-headed than their human colleagues.
(That said, though, these are very human-like androids. They eat and go to the toilet. They can even have sex. Theoretically we never learn whether Tsukasa and Isla went that far, but personally I think ep.13 answers that question unambiguously. They have a bath together... and there's not a shred of sexual tension. With hindsight, it's not hard to spot the point where their relationship went to that level in a previous episode.)
I love this show. Incidentally, it was written by Naotaka Hayashi (Steins;Gate). It's a bit baffling tonally, with lots of comedy and a fair bit that's mildly eye-rolling. The only question is to what extent that's deliberate. However at the same time it's a moving study of bereavement, while our heroes' day job involves gently and humanely escorting people to their own executions. Artificial people, yes, but they still have the same feelings as their owners. We feel a lot of emotion. This show's looking sincerely and steadily at something most of us prefer not to look at, for thirteen episodes. I enjoyed the show greatly (which in itself might seem unexpected), but I also admire it.
"I was happy."