Papyrus is the second major battle of Undertale, following the completion of Swindon area.
The following boss is designed to be played as part of a complete Pacifist guide - in short, going through the entire game without killing anything to unlock a different ending - of which bosses are some of the hardest to beat.
However you're playing, our wider Undertale walkthrough delves into other areas of the game.
Papyrus boss fight
Spare Papyrus for a couple of turns, then Flirt with him, and then attempt to Spare him for the remainder of the fight. The first phase of his attacks will involve some easy to dodge bones - just watch out for the blue ones and remember to stop moving.
After a few rounds he'll turn you blue and you'll drop to the bottom of the box - pressing Up on the controller will make you jump, and the longer you hold it the higher you'll go.
Some groups of bones can be jumped over when you're standing still, but some of the longer groups will require you to move and jump in order to get enough distance to clear them.
Note that you can't die during this fight; if you get down to 1HP Papyrus will capture you and leave you in his shed and you can just try again, and failing several times will result in him just letting you pass unmolested.
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.
Once the fight is over head back into town and go to Papyrus's house. There's 20G hidden in the couch, and you can examine a few items for some more skeleton-based humour.
When you've looked around talk to Papyrus to begin your date - just answer all his questions positively, then when he puts on his Secret Style outfit examine his hat.
Once the bizarre date is over, leave town to the east to Waterfall, the game's third area and another filled with enemies to spare as you continue your journey.