Waterfall

From Snowdin, head east along the river. You'll find a save point and Sans. Talk to him to be invited to Grillby's.

At Grillby's, Sans orders food and tells you about an echo flower that sounds like Flowey, which has been talking to Papyrus.

After the meal, head east from Sans's stand. Cross the river, avoiding tumbling rocks.

In the dark area, you'll encounter a scene with Papyrus and Undyne. Undyne declares she will take your soul. A monster kid, a fan of Undyne, runs ahead. Save your game.

In the next area, use four Bridge Seeds to form a bridge.

Solve the next bridge puzzle: align the flowers parallel with the bottom right shore by placing them at the very bottom of the room and face east/right to access an eastern room containing a quiche under a bench.

To proceed, align the flowers against the back waterfall by standing on a small, two-space jut and facing north.

As you head north, Papyrus calls to ask what you're wearing; your answer doesn't change anything.

In the north room, the Wishing Room, examine the northern wall to open a door.

Battle: Aaron — Flex to out-flex him. Stay high and move side-to-side to avoid his attacks.

Battle: Woshua — Pick 'Clean', then grab his green water. Move in a circle to avoid his attacks.

Ahead, read the signs on the docks about the human-monster war. Step on the last piece of dock on the right to cross the water.

Undyne will appear and throw spears. Run forward, dodging them. Stay low on the docks and move up to avoid the spears.

In the next room, Undyne continues throwing spears but stops when you enter the grass. She accidentally grabs the monster kid.

Save your game. You can return to Snowdin using the ferry monster near the tall grass.

Sans offers a telescope prank. North of here, you can buy Nice Cream. Head south.

Head south, checking echo flowers. At the first fork, go east to a new room. Enter the grass to find Ballet Shoes (a weapon).

Return to the fork and continue south. Papyrus calls again about Undyne knowing what you're wearing.

Head east, then north. Meet Onionsan for amusement, then keep going right.

At the next fork, go east to find a statue. Get an umbrella and place it on the statue to start a music box.

Go back, then north to a room with a piano. Play the melody using the inputs: Z, up, right, Z, down, Z, down, right.

The far wall opens to a treasure. When you try to take it, a dog appears. Release a dog from your inventory; it will steal the artifact and give you Dog Residue.