Created by a single person and heralded as one of the best Metroidvania games on the market, Cave Story has both delighted and frustrated gamers for close to two decades. Its elegant 2D presentation, branching storyline, and oft-steep learning curve makes every playthrough quite an experience.
Offering an expansive lore that goes much deeper than the casual player may realize, Cave Story has three endings. Two are straightforward to achieve, while the third almost requires a guide to reach. We'll explore the process for viewing all three endings in this walkthrough. Be warned, though - getting to that third, secret ending is a doozy of a process.
The Bad Ending
The first ending you can find in Cave Story is by far the worst - but it's also the easiest to discover. To achieve the bad ending of Cave Story you need to make it through the Egg Corridor after it has been trashed late in the game. At the end of the Egg Corridor is Egg No. 00's room. Check out the computer in the room and Kazuma will appear. He'll ask if you want to escape the island with him. Say 'Yes', then follow him outside to the Outer Wall. Speak to Kazuma again to trigger the ending.
It's worth noting that you can skip this ending after saying 'Yes' by simply ignoring Kazuma on the Outer Wall and proceeding up the Wall.
The Normal Ending
Cave Story's normal ending is the sequence players will generally see if they don't know what they're doing. Reject Kazuma's offer to escape, proceed up the Outer Wall, make your way through the events of the Plantation, run through the Last Cave and the Balcony, and survive the three-battle sequence of Misery, the Doctor, and the Undead Core. Beating these three bosses and escaping the island in the aftermath will trigger the normal ending.
The Best Ending
Unlike the first two endings, the best ending of Cave Story requires following a very specific sequence of steps, most of which revolve around the survival of Curly Brace. If you muck up any of these steps the best ending will immediately be cut off from your current playthrough, and you'll need to start over and try again.
The process for reaching Cave Story's best ending is as follows:
Step 1: Avoid Professor Booster
Proceed through the game until you reach Labyrinth B, shortly after the Clinic Ruins. Here you'll see Professor Booster fall into a pit. If you follow him down he will give you the Booster v0.8, which will allow you to zip out of the pit. Do not fall into the pit. Instead, look for a small red mark on the ground, run towards it, and jump as soon as you reach the mark. This will launch you over the pit. If you accidentally fall into the pit you'll need to reload your game and try again.
Step 2: Save Curly Brace
Continue through the Labyrinth until you meet Curly and reach the Core. The two of you will face off against a boss of the same name. After it's defeated the room will fill with water, and Curly will sacrifice herself to save you. Check the ground on the right side of the room for a sparkle. This is the Tow Rope. Grab it, then approach Curly's body. Interacting with her will bind Curly to you.
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Step 3: Continue Saving Curly Brace
Exit the Core and enter the Waterway. Proceed through the currents in the area until you see a door on a ledge, just after a larger room. Leap out of the current before it sweeps you downwards and enter the door. Inside you'll find a bookshelf, a bed, a save point, and a computer. Rest on the bed to place Curly on it, then check the bookshelf for instructions on how to resuscitate a robot. This will wake Curly up briefly, though she will go back to sleep again. You'll then be asked whether or not you want to leave Curly in the cabin. Choose 'No'. She'll get tied to you again.
Proceed through the rest of the Waterway and you'll be separated from Curly. Shortly after this you'll be back in the Mimiga Village, and Professor Booster will show up and give you the Booster V2.0, a more powerful rocket.
Step 4: Retrieve the Ma Pignon
Continue through the game until you reach the Plantation. Fall through the bridge at the beginning of the area and you'll find a small room in the ground below. Curly is on the bed, and she's feeling amnesiac. Speak to her, then to the green-clad Cthulhu watching over her until he mentions a mushroom that can cure amnesia. Make your way back to the Mimiga Village, enter the Graveyard, and boost up to the door above the upper platform.
Inside you'll find a chatty mushroom called Ma Pignon. Ma Pignon will assault you with questions about a Mushroom Badge. Insist on getting it until Ma Pignon hands over the Badge. Inspect the Badge in the item menu, then speak to Ma Pignon to trigger a boss battle. Defeat the mushroom to receive Ma Pignon as an item, then return to the Plantation and give it to Curly. She'll fill you in on some important backstory once her memory is restored. Talk to her again and you'll receive the Iron Bond. Don't forget to grab the Iron Bond!
Step 5: Complete the Last Cave
Proceed through the story until you reach the Last Cave, far above the Plantation. Assuming you've done everything correctly up to this point there is a new version of the Last Cave waiting that is significantly more difficult than the original, and requires precision use of the Booster. Be prepared to work on this dungeon for a while if you haven't already mastered the Booster. Make sure you preserve as much health as you can on the way through, as there's a (relatively minor) boss waiting at the end.
Step 6: Finishing Up
Defeat the three-boss sequence and proceed to the island escape. On your way towards the end of the game, however, you'll want to enter the small house where you could rest and save earlier. Inside you'll find a gap in the floor, and below you'll find both Curly Brace, who provides extra firepower when strapped to you, and the Blood Stained Sanctuary. The Sanctuary is Cave Story's secret dungeon, and completing this level is your final hurdle in reaching the game's best ending.
Be warned: The Blood Stained Sanctuary is easily the hardest area in the game. Not only do you need to overcome a massive number of enemies and obstacles, you'll face off against another boss gauntlet that's significantly more difficult than the last. The final boss of Cave Story has no less than four phases, and if you lose all your lives fighting it you'll have to redo the entirety of the Blood Stained Sanctuary.
In short, getting to this ending will take you a long, long time. Have fun!