The year is 9999 when a young girl named Mion wakes up within the depths of dark ruins, alone, afraid, and confused. Luckily, she is quickly found by two fireflies. One is named Lumen, who lights her way and guides her, and the other is named Umbra, who hides in Mion's shadow and has the power to affect the physical world by traveling through the shadows. Together, the two vow to lead Mion into the world above while Mion tries to piece together her past.
The game utilizes both the front and back touchscreen to control the fireflies (though there is an option to use the analogue stick instead). The front touchscreen controls Lumen, who Mion will try to follow wherever she goes. The back touchscreen pauses the game and turns the world black and blue. During this time, the player controls Umbra who can travel through any black colored structures or items. If said structures have a glowing pink dot, Umbra can interact with this to affect the physical world like causing parts of a structure to collapse to create a small bridge for Mion to cross.
The game features a storybook-like art style very different from previous Nippon Ichi games.
This game provides examples of:
- Adult Fear: Mion is all alone underground when you first find her. That alone would be scary enough, but then you find out there are a whole bunch of shadow creatures looking to kill her as are all the traps and hazards along the way. And it only gets worse from there with her memory fragments revealing that someone broke into her room and killed her while she was asleep and other nasty things you later find out about her past.
- Art Shift: When viewing Mion's past through the Memory Fragments, the art style changes to pixelated sprites.
- Bloodless Carnage: Played with. Many of Mion's deaths can be pretty gruesome, which includes getting crushed by gears and being chopped up by buzzsaws. Luckily, she simply faints when she gets killed, her body never appearing physical harmed in any way. However, her "fainting" is accompanied by the screen being splattered with blood. Very much averted in later memory fragments.
- Crapsack World: A video covering the backstory implies that a war completely ravaged the land.
- The Cutie: We can all agree that it's Mion, but Lumen and Umbra count as adorable too.
- Darker and Edgier: The game is about guiding a little girl through essentially a death trap while several shadow creatures are out to kill her, and it only gets darker as you uncover more of Mion's past. On top of that, it lacks the humor that even some of Nippon Ichi's darker games have. The color palette is also much darker than usual, using mostly greens, greys, browns, and blacks.
- The only things in the game that can be considered humorous are shadow creature reactions to Umbra "poking" them and Mion's silly expression when under the effects of a mushroom.
- Determinator: Did you die 100 times and are still going? Have a trophy titled, "Unbreakable Resolve!"
- Downer Ending: The true ending. Clone Mion kills Lumen and Umbra, Mion's parents, after feeling that she had gone through everything just to be abandoned for the original Mion. The original Mion, with her parents dead and no longer capable of using her body, breaks down and cries. Clone Mion, realizing what she had done, consoles the original Mion and the two enjoy a moment in the sun before their spirits disappear. Basically, everyone died.
- Bittersweet Ending: At the very least, both original and clone Mion reconcile with each other and, after spending well over a millennium in the dark, spend their last moments basking in the sunlight.
- Eldritch Abomination: The creature that chases Mion through the end of Chapter 3-3 and the chapter boss itself.
- Everything Trying to Kill You: Shadow creatures, saw blades, man-eating plants, pistons, and more are quite abundant throughout the world.
- Expendable Clone: The fireflies sure think so. In the true ending as soon as Mion reaches a hidden lab, they tried to get her killed so that the real Mion can posses her. The clone you've been playing as is not amused to say the least.
- Fake Difficulty: In Japan, the game launched with only touchscreen control scheme. The imprecise controls of the touchscreen (Not to mention the player's hand blocking part of the screen) combined with the game's hazards, which require a ridiculous amount precision, made the game nearly impossible. The developer responded by patching in a regular analog control scheme which, while still difficult to play, was better received than the touchscreen control scheme. All releases of the game outside of Japan have the analog control scheme already patched in.
- Fatal Flaw:
- Wrath for Mion, both the original and the clone. Mion gets really angry and vengeful at the thought of being abandoned. The original killed her parents and the clones when she felt like she had been replaced and, likewise, the clone attacks the original and her parents (This time as the fireflies Lumen and Umbra) after having been killed so that the original can take over her body. This anger ultimately ends up killing Lumen and Umbra for good and neither of the Mions can return to life.
- Her parents are very short-sighted being concerned with bringing back their daughter in any form, no matter the cost. Creating a clone to cope with losing Mion is not necessarily a bad thing as they were unaware that the original Mion's soul was watching over them the entire time, but later leading the clone safely through the underground ruins just to get her killed and bring back the original definitely is. Their inability to understand that their cloned Mion was just as real as the original ultimately got them killed for good.
- Gainax Ending: The normal ending. Just when Mion is about to go outside, she becomes very weak to the point of having to crawl out the door. Then, just barely a few feet from the door, she plummets into a giant pit. When she finally wakes up, she finds herself in the very same ruins in which she started her journey and must go through it all over again (thankfully, the player can just got to stage select).
- Department of Redundancy Department: The game's title, htoL#NiQ, is a stylized version of the Japanese title Hotaru no Nikkinote . This translates to "Firefly's Diary", which makes the English title Firefly's Diary: The Firefly Diary.
- Hates Being Alone: Mion, both of them. They cannot stand being abandoned and will react violently if they feel that they have been.
- Horned Humanoid: Mion has tree branch-like antlers. They are what distinguish her as a Mion clone. When the original Mion possesses the clone body, the antlers fall off.
- Irony: Lumen can manipulate plants, while Umbra primarily interacts with machinery. In life, their roles were exactly the opposite.
- Kill the Cutie: This game does this A lot to Mion, be it by monsters, machines, or other deadly things.
- Magic Mushroom: When Mion touches a mushroom, it gives her a dazed look and sprouts a giant mushroom on her head which she can glide with. It also makes her run away from Lumen, reversing the game's controls.
- The Many Deaths of You: Courtesy of Mion being a One-Hit-Point Wonder. There are even trophies for dying 10 and 100 times, respectively.
- Meaningful Name: Lumen and umbra mean "light" and "shadow" in Latin.
- Minimalism: Compared to other Nippon Ichi games, htoL#NiQ has no spoken dialogue at all (if Mion does speak to anyone, it's rendered as a speech bubble with a picture), similar to indie games like Limbo.
- My God, What Have I Done?: Clone Mion's reason for consoling the original Mion after she killed her parents.
- Nintendo Hard: There are plenty of things that can easily kill Mion and she barely has any way to protect herself. You'll definitely be seeing her die a lot.
- One-Hit-Point Wonder: Mion dies in one hit against basically anything in the game. When controlling Lumen alone, touching anything solid can kill her (justified in that the walls are electrified and Lumen can't fly around them).
- Replacement Goldfish: Mion. Both of them. The spirit of the original Mion, feeling that she had been replaced by her clone, kills her parents and her dog in retaliation (at least in one interpretation). Then in the true ending, the clone Mion, feeling that she had been abandoned by her parents for the original Mion, attacks them in retaliation. It doesn't end well for either of them.
- Unexpected Gameplay Change: The boss battle of Chapter X is a tower defense game.