Important Notes - Undertale

Overview

Undertale is a game that reacts to every decision you make. What seems like a small action—sparing a monster, opening a present, or even dying—can have permanent consequences for the world and its characters. This guide covers crucial warnings, missable content, difficulty surprises, save file pitfalls, and things many players wish they knew before starting. Read this before you commit to a playthrough to avoid frustration, missed storylines, or accidentally locking yourself out of the ending you want.

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Irreversible Choices and Endings

Undertale has three main routes: Pacifist (spare everyone, befriend all major characters), Neutral (some kills, some spares), and Genocide (kill every monster you encounter).

Once you kill any monster, you are locked out of the True Pacifist ending for that run. To get True Pacifist, you must never earn a single EXP (Experience Points) or LV (Love/Violence Level) from killing a monster. This includes bosses like Toriel. Sparing everyone is mandatory.

True Pacifist requires more than just not killing. After a Neutral run (even one where you spared everyone), you must reload your save and complete specific errands: give Undyne water, hang out with Papyrus, and visit Undyne at her house after becoming friends. Only then can you access the True Lab and get the true ending.

Genocide run permanently alters your game. If you complete a Genocide run, even after resetting, the final boss of that route leaves a "soulless" mark that changes the ending of future Pacifist runs unless you perform a True Reset (which is only possible after finishing True Pacifist). The game remembers every action.

Resetting vs True Resetting: A normal reset from the menu does not fully clear the game's memory. Flowey will acknowledge your previous actions. To completely wipe the slate, you must complete True Pacifist and then select "True Reset" from the title screen. This erases all files, including any Genocide corruption.

Missable Content and Secrets

Early Game Missables

Toriel's Kitchen Phone: After the first encounter with Toriel, go back to her home and use the phone in the kitchen. Calling it gives you a hidden conversation and the "Pie" (if you didn't take it earlier). Missable once you leave the Ruins.

The Cell Phone Screenshot: In the room with the dummy before Napstablook, interact with the dummy multiple times to get a "Screenshot" item. Useless but humorous.

The Dog Shrine: In Snowdin Town, walk behind the inn to find a hidden shrine. Giving items to the dog can reward you with the "Torn Notebook" and "Manly Bandanna." If you miss this before leaving for Waterfall, it's gone until a new playthrough.

Mid-Game Missables

Papyrus's Date/Spaghetti Cup: If you befriend Papyrus, he will invite you to his house. Accepting gives you the "Spaghetti Cup" and unlocks the key for his special attack during his fight. If you refuse, you miss the item and a heartfelt scene.

Undyne's House in Waterfall: After sparing Undyne, you can visit her house. If you have given her water during the heat-of-the-moment chase, she will be there. Inside, you can fight the "Moldsmal" in the refrigerator—missable if you don't go.

Gaster's Followers: In Hotland, there are hidden rooms with NPCs who hint at the character W.D. Gaster. They appear only if your game's "Fun" value is set to a specific number (which changes randomly on each playthrough). You cannot force them without save editing. If you see a room that should contain something but it's empty, your Fun value is wrong.

Late Game Missables

True Lab Completion: The True Lab is only accessible after finishing a Neutral run where you spared all main characters (Toriel, Papyrus, Undyne, Alphys) and did not kill any monsters. If you killed even one monster, you must complete that Neutral run first, then reload and make the necessary friendships.

The Sans Secret Boss Room: In the Genocide route, after defeating Sans, you can find a hidden room with the machine that supposedly creates timelines. This is only accessible in Genocide and contains lore items.

Difficulty Spikes and Boss Strategies

Toriel (First Boss): Very easy. Defeat her by either fighting until she reduces your HP enough to make her spare you naturally, or by using the "Spare" command after she lowers her guard. Common mistake: Killing her in one hit with a high ATK weapon. If you do, you lose the Pacifist ending permanently for this run.

Papyrus: Easy if you understand his puzzles. Use the "Flirt" option to speed things up. He is always spares after a few turns.

Undyne (Neutral Route): Moderate difficulty. Her attacks are fast but predictable. Stock up on healing items from Snowdin. Use the "Turn" tactic: when she is about to attack, switch to the fight menu quickly to dodge.

Undyne the Undying (Genocide Only): Extremely hard. She has devastating attacks, reduced invincibility frames, and heals herself. You must max out your HP, LV (19-20), and equipment. Learn the attack patterns—many involve circling projectiles. Use the "Burnt Pan" weapon and "Stained Apron" armor for extra healing. Expect 50+ attempts.

Mettaton EX (Neutral/Pacifist): A tricky rhythm-based battle. Memorize the four attack phases. Use healing items liberally. If you fail the cooking show, reload.

Asgore (Neutral): Hard for new players. He attacks quickly and has a devastating trident slash. Use the "Heart Locket" and "Worn Dagger" (from Toriel's house) if you want a harder fight? Actually, bring the "Empty Gun" and "Torn Notebook" for easier dodge. He also heals with the soul shards—make sure to absorb them before he does.

Sans (Genocide Only): The hardest fight in the game. He has only 1 HP but attacks relentlessly, ignores invincibility frames, and uses Karmic Retribution (poison damage). You must practice his patterns for hours. Prepare with the best equipment: "Real Knife" and "The Locket" (or "Stained Apron" for healing). Use the "Sea Tea" to speed up your soul movement. This fight will test your patience.

Asriel Dreamer (True Pacifist Final Boss): Challenging but fair. His attacks are long and have multiple forms. Use the best healing items and the "Empty Gun" to finish quick. The boss is meant to be overcome with determination, not grinding.

Grinding: When and Why It Matters

Genocide Route: You must grind to a specific kill count in each area before the boss appears. You cannot accidentally trigger Genocide; you must intentionally kill every monster in the area except the boss. The game will announce "But nobody came." If you kill too few, you'll get the normal boss. The required thresholds:

Gold Farming: If you want the "Temmie Armor" (which costs 9999G), you can farm gold by repeatedly battling the "Froggit" or "Moldsmal" with the "Dog Residue" trick? Actually, a better way is to use the "Secret Squid" in the trash can in Hotland? No, the simplest gold farming is to sell the "Manly Bandanna" and "Torn Notebook" from the Dog Shrine, or repeatedly fight the "Tsunderplane" in Hotland. But gold is rarely needed except for the armor, which is only useful for Genocide route (to survive Undyne and Sans). For Pacifist, you can get by with free healing items.

LV Grinding: Do not grind LV unless you are on Genocide. Higher LV makes you more powerful but also corrupts your soul. The maximum LV in Genocide is 20 (from Sans). In Pacifist, you remain at LV 1 (if you never kill) or low levels from accidental kills.

Save Management and Resets

Manual Saves: You can save at glowing save points (stars). The game autosaves at many checkpoints, but it's best to always manually save before difficult bosses or risky areas.

Resetting: Pressing "Reset" from the title screen reloads your last save. It does not undo any actions that are already stored in the system file. For example, if you killed Toriel and then reset, the game will remember that you killed her (Flowey comments). To truly start a new story, you need to delete the save files manually or perform a True Reset after True Pacifist.

File Locations: Save files are stored in:

Important: Do not delete or modify the `System Information` file that tracks kills across playthroughs. Doing so can cause the game to malfunction or lock you into a corrupted state. Only do manual deletion if you are sure you want a fully fresh start.

Using Multiple Saves: You cannot have multiple profiles without manual file swapping. To play a different route simultaneously, copy your save folder to a backup location and restore it when you want to switch.

Common Regrets and Tips to Avoid Them

Regret: Killing Toriel. It's the most common early mistake. She gives you multiple chances to spare her. Wait until her dialogue changes to the point where you can select "Spare" until she shows mercy. If you attack, you kill her in one hit. To avoid this, never use the FIGHT command against her until she is ready to be spared.

Regret: Not befriending Papyrus. Right after defeating him, call him on the phone (go to Snowdin Town and use the phone near the inn) to initiate his hangout. If you move to Waterfall without doing this, you miss the chance. Always befriend Papyrus immediately.

Regret: Accidentally killing a monster. If you accidentally kill a monster while trying to spare, do not continue. Immediately reload your last save. If you save after the kill, that run is compromised for True Pacifist. Reload before any boss or major event.

Regret: Missing the Undyne date. After sparing Undyne in Waterfall, you need to give her water during the chase. Then, after the fight with Mettaton EX, go to her house in Waterfall to have a date. If you skip visiting her, you cannot get the True Pacifist ending.

Regret: Going Genocide first. Many players try Genocide out of curiosity and later regret it because it taints future save files. The Genocide route is also the hardest and most emotionally draining. It is best experienced last, after you have fully explored the other routes.

Regret: Not exploring every room. Many secrets are hidden behind invisible walls or require backtracking. Check every nook and cranny, especially in Snowdin and Waterfall. Use the Cell Phone to call Papyrus or Undyne for hints and entertainment.

Regret: Overlooking the importance of the Fun value. The Gaster encounters are random, but they add deep lore. If you want to see them, you can try to set your Fun value via save editing, but be warned: modifying saves can cause glitches. It's often better to enjoy the mystery naturally.

Regret: Selling key items. Some items like the "Mystery Key" or "Temmie Flakes" have hidden uses. Do not sell anything unless you are absolutely sure. The "Dog Residue" can be used to create infinite amounts if you combine it with alchemy? Actually, it's more of a joke item. But never sell the "Empty Gun" or "Cowboy Hat" until you understand their purpose.

Online Etiquette and Anti-Cheat Notes

Save file sharing: Sharing your save file online might spoil the ending for others. If you send someone your save, they can load it and see your choices. Be careful.

Modding and cheats: Using third-party tools to alter gameplay (like infinite HP or forcing routes) can corrupt your save file. The game does not have anti-cheat, but it may break many scripted events. If you cheat, you will likely miss the emotional impact the game is designed to deliver.

The "Fun" value: Some players try to manipulate the Fun value to force rare encounters. This is possible by editing `undertale.ini` but is not officially supported. If you do, make a backup first. Also, changing the Fun value after a certain point may cause game-breaking behavior (e.g., rooms not loading).

Speedrunning: Speedrunners use specific glitches like "Save Corruption" to skip sections. This is fine for competitive play, but for a first-time player, it will ruin your narrative experience. Avoid following speedrun strategies on your first few playthroughs.

Final Advice

Listen to the game. The characters often hint at consequences. If Flowey tells you to spare someone, believe him. If Sans tells you that you should be careful, pay attention.

Play blind. The best experience comes from discovering surprises on your own. Only consult guides after you have completed at least one run.

Expect the unexpected. Undertale breaks conventions deliberately. Don't assume you know what will happen next. Your actions matter, and the game will remember.

Good luck, and remember: your choices change the world.