- Head east after you’ve dealt with Papyrus, one way or another. A short way into this area you’ll find a save point, along with a few NPCs. If you didn't kill Papyrus you'll find Sans sitting here, and he'll ask if you want to hang out; agree and you'll get jumped to Grillby's. (This is a prerequisite for something later, so you may want to agree.) One screen east of here is another Box for storing your stuff.

- There’s a rushing waterfall ahead, and you need to cross while avoiding the hunks of falling ice that try to rush you onto the platforms below. Weave through them to get to the other side. If you brush up against the waterfall you can find a hidden room containing an Old Tutu.

- Things get a bit hairy in the next room, though only from an atmospheric POV. There’s another save point here, as well, which you can use if you felt the last one was entirely too remote.

- There are Bridge Seeds in the next room. Pick ‘em up and toss them in the water to the right. Continue east and, in the next room, you’ll start running into enemies:
  • Aarons. These sea horsies like to flex, and will use a series of beefy arms to try and hit you. They'll also sweat all over you. Scroll to the top of the box to more easily avoid the arms; stay near the bottom to avoid the sweat. You can get rid of Aarons by Flexing until they flex themselves into oblivion.
  • Woshuas. I don’t even know what these things are. Woshuas like to clean, and will hit you with bouncing soap bars and spurts of detergent. Hide in the corners either way. (The green detergent spurts will heal you, so go for ‘em if you think it’s safe.) Get rid of Woshua by asking for a Clean, then moving on to Spare.
- This larger room contains four Bridge Seeds and a Bell Blossom. Ringing the Bell Blossom will reset the Bridge Seeds. In order to create proper bridges the Bridge Seeds need to all be arrayed in adjacent fashion, so you need to set up four in a line without fail. This makes getting across the watery gap in the north trickier than it looks - though all you need to do is deploy the Bridge Seeds towards the water fall, from the third conspicuous jut of land from the top. Look at the picture below to see what I mean. (You can also use the Seeds to reach the sign in the south for some lulz.)

Bridge Seeds at work in Undertale.
- (Before leaving this area, deploy the Bridge Seeds along the southeastern patch of water in such a way that you can step off the land and onto them. There's a secret path to the east of this water that leads to a bench, under which you'll find an Abandoned Quiche. Poor thing.)

- There’s a telescope in the next room. Look through it and you can see ‘Check Wall’ through the scope by moving it around a bit. It refers to the conspicuously-blank wall down the northern dead end from here.

- The next room contains a series of wall plaques, as well as a raft on the east end of the dock. Hop on and it will spirit you across the water. Make sure you’ve healed up…

- … because the knight you saw earlier will start hurling spears at you in the next room. Dash your way east, avoiding spears as best you can. If you get hit by one you’ll be thrown into a short mini game where you need to avoid flying spears. I found it easiest to walk low, wait for the spears to fly, and then move upward while continuing east. Go low again once the knight is prepping for another throw. Don’t get caught too often (or just come through at full health) and this shouldn’t be too hard.

- After a quick cut scene you’ll find a save point. (One day, mouse… one day…) In the next room is a Box; beside the Box is a side room with another Box, as well as the Nice Cream vendor. If you buy a Nice Cream you can take a Punch Card from the Box. Get three and you’ll earn a free Nice Cream. What… what a deal. (If you're carrying an umbrella, which you can get soon, you'll get a discount down to 15 gold, from the vendor's current 25 gold price.)

- To the east in the previous room is a duck. Yep. You can later use this duck to backtrack more quickly.

- Head south. Walk along the pathway in the next room until it splits to the south and the west. To the west is a single bush; hunt around inside it and you can find Ballet Shoes. Backtrack and take the southern path.

Onionsan. So kawaii.
- A big dude named Onionsan lives in the next area. He’ll follow you around and be generally friendly, and if you backtrack once he’ll have something new to say. Be nice to him and he’ll follow you through this room every time you come through. (Probably not often.) He is, as far as I know, utterly useless.

- There are new enemies in the next area:
  • Shryens. Lilting, shy crooners, Shyrens shoot musical notes at you which can be easily avoided by remaining at the bottom of the box. You can safely get rid of Shyrens by Smiling and Humming in that order, then Sparing them. Shyren attacks get more vigorous when they’re happy, so be careful.
The path splits here. If you head north you’ll find a piano, with instructions on how to play… though you don’t have a song to play. You’ll get clues for it in the next room, but if you want to bypass the detective stuff, the song is as follows: Z, Up, Right, Z, Down, Z, Down, Right. (Z is just a neutral button press. You need to hit Z plus the direction for the rest.) What you find beyond… well, you’ll see. Ultimately you get Dog Residue from this weird, weird transaction. Dog Residue can be sold, and you'll later gain a means of replicating it endlessly, assuming you don't get rid of it all at once.

- To the east from the split is a statue, and beyond that a bunch of umbrellas. Put an umbrella on top of the statue and the falling rain will create the song you need to play on the piano. (If you want to figure this out on your own, just remember that you’re only playing the first eight notes.) You can grab another umbrella to advance with a chatty monster at your side, as well. At the end of this path you can return the umbrella, if you like.

- There’s a save point in the next room. Use it, ‘cause the knight is going to be chasing you again in the next area. This time you need to avoid spears that pop out of the floor from blue portals. This is easy so long as you take your time and stand still when you’re caught between spears. The area you need to traverse is quite large, and full of dead ends; head east until it starts to branch out, then start wandering to the southeast. Eventually you’ll hit another room, along with a dead end, and a cut scene will drop you into a new area.

- To the east of where you land is a save point. Wade through the heaps of junk to the northeast and, along the way, you can find a cooler containing several Astronaut Foods. Nearby is a Dummy; punch it and you’ll get some grief.

The Mad Dummy comes at you, bro.
Mad Dummy

This is a weird two-part battle. The Mad Dummy employs a series of fellow Dummies which will fire what appear to be cotton balls at you. These aren’t too difficult to avoid - but that’s all you’ll seemingly get to do, as no Actions work on the Mad Dummy, and physical attacks are useless. The trick here is to get the smaller dummies to fire at the Mad Dummy, as their bullets fly out of the box. This gets trickier to do as the battle wears on, since the Mad Dummy starts to move around and won’t allow you to get in as many shots, so stick to the top of the screen and try to stay with it at all times.

Eventually you’ll do enough damage that the Mad Dummy will start to pull out robotic puppets. These launch missiles instead of cotton fluffs, and the missiles are much harder to avoid. Stay away from the robots before they launch, then guide the missiles around in one or two loops. Once they’ve looped enough times the missiles will fly off in whatever direction they were pointed, preferably up towards the Mad Dummy. Do this enough times and the battle will end.

- The path splits to the north as you follow Napstablook. To the west is the duck you saw earlier; he’ll ferry you back to the beginning of Waterfall. (Or close enough.) There are houses to the northwest and north; the central path leads to Napstablook’s home. There’s plenty to do if you check all of the appliances and keep talking to Napstablook, though little of substance. East of his home is a snail farm, as well as the epic game of Thundersnail.

Thundersnail. There is no sport more exciting in Undertale.

This is, weirdly enough, a decent way to make money if you feel like wasting a lot of time. Thundersnail is a race between three snails from one end of a track to the other. Your snail is always the yellow one up top, and it’s always the slowest. In order to make your snail move more quickly you need to encourage it with the X button. Every time you do it will move just a little bit faster… but only if you don’t encourage it too often. If you ram the button you’ll paralyze the snail, and may set its shell on fire. Ouch.

The key here is to carefully time your encouragements. For the first half of the race you should encourage the snail every two to three seconds. Once it reaches the halfway point, let the snail run the rest of the course. It should be fast enough to outpace the red snail by a small margin and win the race. If your snail stops completely, you’re tapping too quickly.

Entry into a Thundersnail race is 10 gold. If your snail wins you’ll receive 9 gold, because Napstablook needs to earn a profit. Lame. If, however, your snail loses by a tiny margin - it basically has to be on the checkered strip - you’ll get 30 gold, because Napstablook is embarrassed by the mix-up. Use encouragements to temporarily slow down your snail enough for the red one to win. Get good enough at this and you can earn gold relatively quickly.

- Head east of the save point after you’re done mucking about with Napstablook. There’s a Box on the next screen, and beside it is a store. The storeowner has lots to say, so Talk through all of his conversation options to learn more about the locals. The weapon and armour he sells (Cloudy Glasses and Torn Notebook) are statistically inferior to your current stuff, assuming you’ve been upgrading, but they’ll allow you a longer period of invincibility in battle whenever you get hit by an attack. I prefer more defence, but it’s up to you. Regardless, stock up on healing stuff - the next boss battle is loooong, and you’ll want to be able to heal.

- The next area to the east contains an unlit path which you can make appear by interacting with the clusters of mushrooms. Simple enough. A new enemy appears here:
  • Temmies. Temmies are adorable little dog things that attack by trying to walk on you. Stay between their legs and you’ll do fine. If you Flex in front of a Temmie you'll bring an Aaron into the fight, after which you can Spare the Temmie.
  • Moldbyggs. Masquerading as Moldsmals at first, Moldbyggs rise up after an interaction. They fire projectiles at you, and are no big deal. You can Unhug them to get a Spare win.
Once the path here seemingly ends, make your way back to the start to find another hidden path that has become illuminated that will take you west. From here you need to muck about in the dark, looping north and then back south, on your way to the east. There’s a bit of stumbling, but it’s not too difficult to make your way, and illuminating all of the mushrooms will light all of the paths. Convenient! If you head to the southeast corner you can also find a path that leads to an optional side area, the Temmie Village.

Temmie Village

What a useful place. Temmie Village is filled with Temmies, as you might expect, and they're a pack of vapid idiots. Have fun! Talk to whomever you like, but the only person of real use is the dude in the 'TEM SHOP'. He's the only vendor you'll run across who will buy your stuff, albeit at some bizarre prices. He tends to purchase one-off equipment items and stuff from the Ruins (Spider Cider, Monster Candy, etc.) at much higher prices, and occasionally will really want an item you're offering him - to the point that you can drive up the price. Don't push him more than once, or the price will drop back down permanently.

The vendor's items are almost universally useless, save one: sending him to college for 1,000 gold. This seems like a waste of money, but if you do this you'll unlock the temy ARMOR!!! item for purchase... for 9,999 gold. Eesh. The price comes down every time you die, though, and will eventually be cost-effective. It's a fantastic piece of armour if you bother to save up. (I find buying Cloudy Glasses en masse from the turtle vendor and selling them to the Temmie to be a relatively quick way of doing this, as you'll get a 20 gold profit on each one, and potentially more.)

- Back to Waterfall. To the east is a maze-like area with purple crystals. The longer you linger here, the darker it gets, until you’re stuck wandering blindly down the paths. The trick here is to interact with the small, blue lamps that appear intermittently along the way, as they’ll light the area and allow you to advance. You want to head to the northeast to leave.

- In the next area you have the choice to save the monster who’s been tailing you this whole time or leave him to his fate. Turn back west and you’ll find a new path to the north after this encounter. In the next area you’ll have a conversation that changes depending on whether you killed Papyrus or not, though it ultimately results in the same battle. Save up and approach the cave to fight.

Undyne brings it on. Will you make a friend, or is she going down?

The most difficult creature you’ve encountered thus far, Undyne is no pushover. She can’t be Spared immediately, like most bosses, and has lots of health. You can either defeat her with Challenges or physical hits, both of which have roughly the same effect on her attack patterns. You can expect the following when battling Undyne:
  • For the first little while Undyne launches darts at you. Use the arrow keys to redirect the shield in front of your heart to ward them off. These get progressively faster and faster, and when she says she’s done ‘warming up’ she’ll launch a spear at you that you can’t block. You’ll have to move out of the way.
  • Once your heart is red again you’ll have to dodge spears similarly to previous non-encounters with Undyne. Move in loose circles for the easiest experience.
  • Occasionally you’ll also be stuck in a small box with spears launching out of the ground at you. Stay near the top and move into lines that currently have no spears poking out of the ground. Easy enough.
  • Late in the battle Undyne will introduce a new kind of dart to the first set of attacks. These yellow darts will ultimately attack from the opposite side of where they’re launched. This is tricksy, but these darts usually correspond with other, normal darts, so you don’t need to pay attention to them much. (Still, keep your health up during these sections. There’s one phase where she only uses yellow darts. Ridiculous.)
If you decide to go the killing route, note that Undye will pull herself back together once you deplete her health. She’s weakened by this point, though, and each attack will get weaker and weaker until she ultimately, gruesomely goes down. Yeesh. Not difficult, but keep your health up anyway. If you don’t kill her, but Act her into submission... which is to say, just avoid her attacks for a loooong time... she'll become repetitive in her dialogue (...) and you can run to avoid killing her. Doing this will lead to a high-speed pursuit into the next area, though you can effortlessly run from this point on.

Once Undyne is out of the way, one way or another, the path will be open to your next destination: Hotland. If, however, you spared both Undyne and Papyrus, as well as hanged out with Papyrus in Snowdin post-battle, you can head back to Undyne's house (it's just west of Napstablook's, and predictably looks like a fish) and become 'besties'. This will earn you a joint phone number between Papyrus and Undyne.