If you’re anything like me, this will simultaneously shock you, warm your heart, and leave you laughing at its convoluted brilliance.
The video is about 13 minutes long, but the payload comes in the last 30 seconds, where balloons are displayed on screen while music plays in the background. What’s so special about that? The image and music featured do not exist anywhere in the game—they were manually programmed to appear in it by taking advantage of game bugs shown in the first 12 minutes of the video. Assuming you know how to program in the gameboy’s machine language, you can turn Pokémon into any program you want.
The video itself is somewhat tedious to watch, because setting up the “bootstrap” program which allows one to write arbitrary programs was accomplished by acquiring a specific sequence of items in exactly the right quantities. For example, about two full minutes are spent doing nothing but buying lemonade (which can only be purchased one at a time). In addition, for much of the video it is difficult to determine just what’s going on; one gets the impression that the game itself is similarly confused!
For more detail, see this post by the author. My hat’s off to you, Robert McIntyre.