The game begins with a brief scene of dialogue, and then cuts right to the opening level. You begin in a safe room, with two devices and a door. A very quick check reveals that the floppy disk saves your game, and the heart monitor restores your health. While neither of these are necessary from the very beginning of the game, it establishes what will later become very important elements of the game's "safe zones."
Exiting the room takes you to the first cave. On your right, the way is obstructed by odd-looking blocks and a bouncy little blob monster, neither of which can be interacted with or destroyed at this stage. You have no weapons!
The right side accounted for, going left reveals bright red spikes and bats that fly in vertical patterns. Just out of sight is a small box with a heart in it. The bats and spikes are both easily bypassed by jumping, a familiar and simple mechanic that comes naturally to most gamers.
After jumping past the bats and spikes, you can reach the heart box. By interacting with it, you find out it is a "Life Capsule", which increases your health capacity and familiarizes you with this item. The reward of additional health is also rather straightforward, but makes these items very desirable.
Continuing down the tunnel and to the right, we see more bats and water. The bats are again easily dodged, but we also learn the mechanics of water in the game: It slows you down, and brings up an "Air" timer. While there's no real risk of drowning unless you made a concerted effort toward it, this is a helpful little tidbit to keep in your memory for the future.
At the end is a large lion's head in a dead-end. Being different from everything else, we naturally assume it is some kind of entrance or other interactive thing.
Inside it turns out to be somebody's house, and the somebody is asleep. At the end of the room is a chest. Talking to the sleeping person yields no useful results, as apparently this is a very heavy sleeper.
Bypassing the sleeping person to rifle through his belongings, on the other hand, gets us a sweet new gun, the Polar Star. The game only using 2 buttons, Z to jump and X to interact, makes figuring out how to shoot a pretty quick process.
Returning to the dead end, we have no option but to backtrack, shooting a lot of bats along the way. Exacting revenge on those cursed bats gives us hearts, which restore lost health (which was probably from the bats)
And little triangle things, which increase your weapon's power. While the first cave doesn't contain enough triangles to learn this immediately, they appeal to our kleptomania and thus you want to collect them anyway for the future.
The gun is also good for shooting those weird rocks from earlier, opening up the right path that was initially unassailable.
Here we learn another lesson about the game: Pay attention, because some of the doors will try to straight up kick your ass unless you kill them first. Approaching this door causes an eye to open, and if you touch the door it damages you. Unfortunately for the door, guns do not require touching what you shoot at.
After killing a door (trust me that's not the weirdest thing you do in this game by a long shot) we can go through, and begin our adventure in the world of Cave Story!
As a tutorial level, while not providing any kind of direct dialogue to tell you what to do, this level takes a more natural approach, letting you learn the core mechanics of the game such as exploration, returning to previous areas to try new things, shooting guns, and finding collectibles, while not being incredibly overwhelming right off the bat.
Here is a simple map of the level:
