Mighty Hoax is the second episode of Angry Birds, which contains two themes with 21 levels each. Much like its preceding episode, it takes place once again in the vast plains. In this chapter, the Pigs try again by stealing the birds eggs, replacing them with cardboard cutouts, and using cardboard decoys to get away.
When the Birds complete Theme 4-21, they discover the King they defeated was in fact a cardboard cutout, and that the real King is still out there. Later, the Birds finally succeed after destroying another cardboard cutout in Theme 5-21 and retrieve the eggs. The real King Pig retreats while the Birds celebrate their temporary victory.
In contrast, this is also the only episode where the King escapes without injury rather than getting brutally beaten by the Birds.
However, in Angry Birds Trilogy, that ending changed. The birds got their eggs back, just like in Classic, but instead of escaping the birds, the King Pig winds up lying in pain face down with the cardboard bush under him, with Bomb the Black Bird standing on top of them.
This is the final episode to contain 21 levels per theme.