First, make sure you've done the date with Papyrus in Snowdin Town, then head east out of the village.
Keep heading east and you'll come to a save point. Talk to Sans and accept his invitation to Grillby's and listen to his story, then return here and keep heading east.
Cross the waterfall avoiding the rocks that will push you off the edge, and enter the next area.
It's dark here, and when you reach the patch of tall grass you'll overhear a conversation between Papyrus and Undyne. Once the conversation is over move to the right and a small monster child will appear, then after a brief conversation with you will run off.
In the next area you'll find four small plants in the ground next to the river.
Pick them up one by one and use them to build a bridge across, and you'll see another four just to the north. Pick these and stand on the outcropping that pokes two squares out into the water and put them down to the north to form another bridge.
After you've crossed the bridge Papyrus will call and ask what you're wearing - the outcome is the same whatever you answer. Keep heading north and you'll come to a room with a telescope. Look through it and inspect the bottom right corner of the room for a clue about where to go next.
Examine the wall in the north of the area to find a secret door. The signs along the dock ahead provide more backstory, and when you reach the end of the dock you'll be carried across into the darkness.
Undyne will start throwing spears at you from above, so run to the right and keep to the bottom of the path, moving upwards every time she throws them to avoid being hit - if one does catch you you'll have to avoid them in a mini combat window.
Make your way into the tall grass, and after a brief case of mistaken identity you'll be in the clear for now.
Sans is waiting in the next area with another telescope, but this one is only there to make you look stupid so feel free to ignore it. In addition to the save point there's a Nice Cream seller to the north and an interdimensional box if you need to resupply.
Head south and look at the Echo Flowers for more background information. When the path splits take a detour to grab a pair of Ballet Slippers to increase your armour, then resume the journey south.
After Papyrus calls with some "good" news head east and then north, the east along the edge of the water. Onionsan the octopus will have a few words for you as you go, but he poses no threat.
Statue and piano puzzle solutions
Enter the next area, ignore the path to the north for now, and keep heading east.
Ignore the statue for now and enter the next area and grab an umbrella, then return to the statue and give it some shelter from the dripping water and it will start playing a tune. Head back to the path to the north and enter the next room to find a piano.
You need to play the tune that the statue gave you - you have five notes available, one from just pressing the button, and four from holding a direction and pressing it. (If you're not musically inclined the tune is none, up, right, none, down, none, down, right.) Head through the door.
When you try and grab the treasure you'll be told you have too many dogs in your inventory. How it got there is unclear, but sure enough you seem to have picked one up, so drop it.
It'll make off with the treasure but leave behind some Dog Residue; this is odd stuff, but can come in handy later: using it produces more Residue plus a Dog Salad healing item, so you'll never be stuck without a way to heal, and you can also use it to turn a tidy profit shortly.
Backtrack past the statue, keep going east, and you'll eventually meet up with the little monster again. Carry on to the east, and when you reach a ledge the kid will help you up. Make your way into the next area, save your game, and then go east again.
Darkness will descend, and Undyne will reappear with her spears. This time she attacks from below, but the danger areas are clearly highlighted so whenever she makes her move stand on a safe spot and wait.
There are multiple paths through the area - you need to head in a generally south east direction, but your exact route will be dictated by the spears.
You'll eventually reach a wide platform that heads directly east, at which point the attacks will stop - Undyne has you trapped! She'll cut off the end of the pier and you'll fall into the void below.
When you awaken you're in some sort of sewer. Head east examining the piles of detritus as you go to scavenge some food, save at the save point, and then head north. When you try and leave the nearby dummy will come to life and you'll have to battle your way out.
Mad Dummy boss fight
You can't Act your way out of this fight, and even if you're not playing a Pacifist run you can't attack him either. The only way to damage him is to use his own attacks against him - lead his projectiles around the box until they're heading his way. Once you've done him enough damage Napstablook will show up to put a stop to the fight.
Once you're out of the sewer Napstablook will invite you to his house - there's nothing much of note to do in there, but he seems like a nice guy.
In the north east of the village there's a snail race to kill some time with, and there's a shop where you can buy a couple of new weapons - even if you're playing a Pacifist run they're worth having as they give you a few more invincibility frames when you take damage in combat.
When you're done in the village exit to the east - there are a few signs here that provide some insight into what's going on.
Playing Undertale? Our complete Undertale walkthrough and Pacifist guide will explain how to finish the game while sparing everyone, covering all areas including the Ruins, Snowdin, Waterfall, Hotlands and The Core and more. You can also learn about how to unlock every Undertale ending and all Undertale Trophies, access hard mode and play the game in the most evil way possible with a Genocide run.
Finding Temmie Village
The next area is very dark, but you can use the mushrooms to light the path ahead.
Your goal is to the east, but if you head south you'll find Temmie Village which is worth a visit; it's a slightly unsettling place, but it contains the only shop in the game that allows you to sell items.
If you've got the patience you can use the Dog Residue and sell the Salads to rack up a decent bank balance, and paying the shopkeeper's college tuition will allow you to purchase the Temmie Armour - it's very expensive, but the more you die the cheaper it gets, so if you can't afford it now you can always come back later.
Once you've left through the main exit from the mushroom pathway you'll be in an even darker area.
Use the lanterns to light your way - unlike the mushrooms they don't stay lit which makes it a little trickier, but there are enough of them around to keep you from being plunged into blackness.
At the end of the next area you'll come across an Echo Flower. Talk to it, and Undyne will appear, but yet again the little monster will save the day. Once you're alone again head west, then take the path to the north - the Echo Flowers here provide some more story details.
Keep following the path and then cross the narrow bridge. The monster will show up again and he'll ask you to be mean to him; it doesn't matter either way, but if you want to help him out then simply refusing to be mean to him is in itself mean, and he'll be happy.
When he leaves he'll slip and fall, and then Undyne will show up. You can run forward and grab the kid or leave him hanging - Undyne will save him if you don't. Regardless of your actions the two of them will leave, and you're now free to finish crossing the bridge.
Keep moving east and you'll come to a large cave, and Undyne is lying in wait, in what is arguably the most challenging fight of the game so far, especially on Pacifist.