Knytt Underground is a side-scrolling exploration platformer with a simple basic goal: the world (as the name suggests a underground cave) is about to die and so the protagonist Mi has to ring the six fate bells. Of course those are divided over the whole world (spanning about 1800 one-screen rooms) so she has a lot exploring to do. Since Mi is mute, she quickly gains two fairy followers which do the talking for her. Those have different personalities and in some conversations she can choose which one should talk, but mechanically this does not make a big difference.
The game is divided into three distinctive chapters which can be played in any order. The first two are relatively linear sections which introduce the the two main characters Mi (can climb on walls) and a rubber ball (can bounce and therefore reach more places). In the third chapter the two characters merge and Mi can transform to the ball at any time - with the abilities of both characters at hand, the player can now explore the world freely. Since this is no metroidvania game, there are no additional powers and items to find, but of course there are still obstacles to overcome in order to open up more paths: NPC quests, platforming and puzzles.
NPCs which block off paths have to be convinced by solving their fetch quests which leads to even more exploring. Platforming consists not only of reaching the next exit point with the characters' abilities, but also of avoiding environmental obstacles which can be static or moving. While Mi has to visit one of the save points to record her progress, dying just sets her back on the same screen to try again. Puzzles utilize special, limited powers which can be found nearby. For example, collecting a green colored orb gives the power to freely fly over the screen for a short time and allows to reach a previous unavailable area. Of course puzzles often require to combine multiple powers. Because the game world is so big, there is a limited fast travel system: when reaching an entrance point to the Disorder parallel world, Mi can use a special crystal (which has to be found first) to enter it. Then she has a limited amount of time to find an exit to another area.