When you begin Megamash, a short Flash video plays of a cart being inserted into the "Nitrome Game System"; you're then offered a (bogus) choice of 7 different games. When you select, the screen fills with a "cartridge read error" message, and the game begins. This is contextualization for the odd mix of gamestyles that follows.
At first, it seems like a simple platformer in which you play a bunny collecting carrots and avoiding enemies. But half-way through the level, you pass through a barrier into a sidescrolling shmup, and become a spaceship shooting aliens with the space bar.
In otherwords, the game is actually a sort of mashup of seven different games, and you pass from one to the other over the course of a level. And indeed -- this is the clever part -- you often need to do something in one "game" in order to advance in the level. Thus, in the rabbit platformer, a crate you need is surrounded by impenetrable blocks; but when you fire across the shmup/platformer boundary, the bullets turn into falling fireballs, which obliterate impenetrable blocks, freeing the crate you need.
The inherent difficulty in each of the "minigames" is tuned low, so that you do not need l33t platforming or shmup skills; rather, the game is essentially a puzzle, in which you must figure out how to combine elements from the different game regions, or move items from one to another, in order to advance.
One flaw: You can get yourself into circumstances from which you cannot recover. However, one of the controls at upper right reloads from the last checkpoint, so this is not as annoying as it might otherwise be.
The controls are a little muddy, but since difficulty is tuned low, this is tolerable; and the nature in which it combines games of different genres is quite clever.