A love letter is sent to a girl who can see the future but has no memory of the past. Despite her foretelling ability, she can't see who sent her the letter but she does know that it will send her on a lengthy journey to find the letter sender. Her friend, an elderly mathemagician, takes it on himself to calculate and map out the journey they're destined to take: one that involves puzzles and riddles that may or may not be solved, mundane and fantastic creatures with a quest of their own, and the titular planetarium where someone — or something — awaits their arrival. And at the end of all this, an overarching message that can be deciphered ahead of time if the person this story is narrated to is observant and clever enough.
Created online way back in 1999, Planetarium still stands out as a highly inventive and unique story-puzzle fusion. The story unfolds gradually over a span of twelve weeks, with one part unlocked for every week, giving the reader-player time to solve the three puzzles given in the newest part. It isn't necessary to solve the puzzles to see more of the story and the puzzle solutions will be available to view at the end no matter how well or poorly you did on them, but successfully solving them fills up your "Table of Solutions" and enables you to solve the Major Puzzle and read the final message of Planetarium before it tells you, just as the girl knew what people were going to tell her before they actually did.
In short, Planetarium is a journey for both the characters and player with them being able to see things ahead of time with the right mindset. It is also a love story and puzzle game rolled up in one, where everything is interconnected by fate and the droll voice of the narrator. It is, to sum it up, in a category of its own.
This story-puzzle contains these tropes:
- Back to Front: Some of the puzzles work this way. Namely, the Major Puzzle is solved by selecting the correct word spheres beginning in part XII and ending in part I. The last part also reveals that if you take the first letter of the animal in each part that's stated to be on a mission, starting from part XI and making your way backwards to part I, you get the contents of the love letter because the letters in the letter were somehow passed on by twelve animals backwards through time to the girl.
- Because Destiny Says So: The main reason the plot begins in the first place is that the girl knows most of what's going to happen in future parts and the mathemagician knows even more than she does after making the proper calculations on his machines. One part even addresses why the girl doesn't try to Screw Destiny by comparing going off the predetermined path to a persistent itch that worsens until you relent and scratch it.
- Blank Book: The atlas of the unknown world in part V is said to be completely blank except for the scale printed on the bottom of every page, for obvious reasons.
- Blessed with Suck: The girl can see her future up to a point, but can't remember anything that happened to her in the past. It's said in the first part that she eventually moved away from her family because they kept on using her as a fortune-telling machine.
- Call-Back: Some of the puzzles require you to check text or images in earlier parts to solve them.
- Crazy-Prepared: The mathemagician is hinted to be this. That, or he just knows how to be prepared from his extensive knowledge and calculations ahead of time.
- Ending tropes:
- Bittersweet Ending: The girl gains normal hindsight and a faithful robot companion, but she has no memory of her mathemagician friend who set this all up to give her a normal life and will never see her again due to being punished by the time authorities to travel backwards in time.
- Ridiculously Human Robot: The letter sender turns out to be a robot boy built by the mathemagician. He's stated to have many characteristics of his creator, including a love of riddles, and is apparently advanced enough to be able to send things or animals backwards in time.
- Swapped Roles: Foreshadowed in part V, where the girl and mathemagician both avoid telling the other that when they reach a line, something about them will become "most strangely reversed".
- Final-Exam Boss: The Major Puzzle is sort of like this. It requires all of the puzzle solutions from all parts to be correct (although you can probably still figure it out if you have just one or two minor solutions wrong), and you have to piece together the clues given to you in various parts to figure out the method to solve it (as you're not even told upfront just what the Major Puzzle is) and then input all parts of the solution in the correct order.
- Foreshadowing: Lots of it, the most prominent being the ghost from the future in part II (who even holds a ball bearing the number of the future part he appears in) and the things the girl and the mathemagician deliberately avoid telling each other in part V.
- Futureshadowing: Some parts give you information about future puzzles and characters before you reach the parts they're in.
- Genre-Busting: Is there a genre for something that manages to be both a (non-mandatory) puzzle game and a story narrative (that can stand on its own) at the same time?
- Hidden in Plain Sight: One of the puzzles in part XI reveals that not only is a mouse hidden in this part, but there's another animal hidden in part II which you most likely hadn't noticed before. You probably didn't see the hidden number in the tree either until told in part VII that the mathemagician was dreaming of it.
- Ironic Echo: Part III mentions that the clocks in the mathemagician's laboratory have two second-hands that pass each other at VI and XII. This occurs to the mathemagician himself in part XI where he's sent back in time to part VI where his earlier self just so happens to be, and From a Certain Point of View he does come back for part XII.
- Laser-Guided Amnesia: Part of the girl's Blessed with Suck package: she sees things when they haven't come to pass yet, but she forgets them as soon as they come to pass. This results in her losing all her memories of the mathemagician at the end when she uses up her foresight and begins remembering things normally, though it's implied that she does retain some subconscious memory of him.
- Merlin Sickness: The girl has a purely mental variant. She's cured of it at the end, but the mathemagician gains a physical variant of it by being forced to travel backwards in time.
- No Name Given: We never find out the girl or the mathemagician's true names. The mathemagician introduces the girl by name to someone else in part VI, but the name he gives for her is left up to the reader-player to decide (literally — it served as a tiebreaker way back when Planetarium was still new and a lot of players were competing to finish it first; it doesn't do anything now, but you can still submit a name if you wish) and it's said that this name may even be a fake one.
- Not-So-Abandoned Building: The titular planetarium.
- The Password Is Always "Swordfish": In part X, you're given a riddle for a password. The answer? Password.
- Red Herring: Some literal ones are shown swimming in the ocean in part VII. Funnily enough, the clue to the Major Puzzle given in their description is actually right.
- Riddle Me This: There's a riddle in every part. Subverted in that solving the riddles isn't needed to proceed to the next part, but they do need to be solved if you're trying to solve the Major Puzzle yourself.
- Rule of Three: There are three puzzles in each part: one a riddle, one a number-based puzzle, and one an either-or question. Additionally, the three-legged triskelion met in part IV has this as his main character trait, giving a riddle in three verses (although the answer isn't "three", a fact that the text lampshades).
- Talking Animal: Two of them show up in part X. They're a fox and a wolf, although you likely knew that already because of references in earlier parts to the puzzles involving them. They also invoke What Measure Is a Non-Human?, with the text mentioning that no matter how humanlike their wardrobe and speech are, the humans they interact with gradually become edgy about being in the presence of too-intelligent animals.
- Temporal Mutability: It's kind of fuzzy how Time Travel works in this universe, but it's said that there's a book of laws against tampering too much with the time stream and that the mathemagician has racked up more than his fair share of violations of these laws. The time agent in part XI sends him back in time as punishment for his numerous violations.
- Timed Mission: Sort of. You can't submit any more puzzle solutions after twelve weeks and your account becomes permanently frozen after another week passes. You also lose your account if you don't log on it for more than ten days. Although you're allowed to make as many new accounts as you want, so losing one account doesn't lock you out of Planetarium forever.