The sequel to Knytt, Knytt Stories, includes a tutorial, a “save-the-world adventure,” and even a level editor. You might be thinking that the level editor will be too hard to figure out -- well it is, at first. But after reading this guide, you should be able to recreate the hardest levels from Mega Man 2 relatively easily.
Knytt Stories doesn't have a specific plot. Instead, each level is its own little adventure. A level called 'The Machine' is included with the game, where you have to stop a machine that draws the life out of the planet. An official expansion pack is also featured at the website with four original levels.
While I can’t comment on the main adventure’s length (haven’t beaten it yet), I will say that based on what I've seen so far, Knytt Stories is worth checking out for anyone with a PC. It's seriously that much fun.
Download Knytt Stories right here.