Flattened London, from the cover and title, provides a mashup of Flatland and Fallen London. It's far more the second than the first, going in more for a Zork I-style treasure hunt, replete with treasure chest to dump stuff in, than any sort of mathematical theory or knowledge. I have to admit I was hoping for the first, but I wasn't disappointed with the second. The items you must find are suitably odd and droll and entertaining, and so it kept my interest quite well. You will probably enjoy it if you go in looking for adventure and not abstract enlightenment. That's not to say it's mathematically illiterate--lines like "Eight candles form an image of a cube" work quite will.
It does seem to have everything: a world through a mirror, a river of death where you play chess, and a mystery and near-conspiracy theories about the third dimension, which is still a touchy subject. And it has good laughs, too. But for pure-puzzle enthusiasts, you may want to know that a lot revolves around finding the right books to read and then following or interpreting instructions--even for the chess! Mapping is a moderate challenge, though it's fun to see the full world being built and all the odd locations. In one case, there's a MasterMind style puzzle. It's more about the story than anything else. And it has some mathy puns in that will make you groan happily.
So if you don't want to approach the trickiness of, say, deriving the Quadratic Formula, then FL will probably appeal. You start off as an equilateral triangle (classes aren't played up as strongly in FL,) and you visit Mr. Pages, a bookseller who wants a book. Clearly for collection purposes--or is it? You seem to have to visit some pretty odd places, including some through a mirror which provide you transport to a flipside. I'm not clear on precisely how the mirror works, but you can only enter it in some places, and it's a handy device for getting out of areas with no way back. It feels just a little illicit, especially as you help others use it before you finally have cause to, yourself.
Things get more illicit with a summoning ceremony, too. To get there, you'll need to go through a maze and navigate some seedy polygons with various different sides, even bringing two together for a common purpose. Many descriptions are funny. There's a very bad painting that hides something obvious after a puzzle is already completed, and it's a nice touch. The different endings for the different hats (there's more than one per hat) you choose at the beginning make things click, too--each profession has a different reason why understanding the third dimension would be useful. And the ending command, well, I can't spoil it, and even if you guess it, it's fully appropriate.
Perhaps one problem with FL is that you may be overwhelmed by a huge inventory--part of that is ameliorated by how only certain items fit in the trophy case (it'd be nice to have a scoring mechanism, even just "1 of 13," to give you an idea of progress, as well as some foreshadowing by your trophy case that this isn't just a treasure hunt.) And the game tries to destroy certain items that are no longer useful. But inventory munging does add a degree of discomfort to an otherwise entertaining and robustly whimsical affair.
And it is a lovely combination of nonsense and speculation that could've fallen apart quite easily. While I had trouble remembering certain hows and whys on multiple play-throughs, I did play through it more than once. And if it is more Fallen London than Flatland, and I played it more for the second, the fun is very real and a good Fallen London advertisement, even as fan-art, for noninitiates. It's the sort of imagination that makes me feel at home, and it doesn't try too hard to be odd. If you wonder and hope it is for you, it is. I didn't find my enjoyment ruined by having to go to a walkthrough.