Unless you really want Jirachi, stay clear of this atrocious excuse for a Pokèmon game.
The point of the game is watching TV. On your Gamecube. With Pikachu. Woohoo.
Initially, you are contacted by Professor Oak, and he wants you to watch Pokemon Channel's various programs on a TV he sends you. You are supposed to review the shows and report to Prof. Oak at the end of each day. The shows come on different disks and are delivered to your door by a Magnemite. As you sit and watch the incredibly boring and pointless programs, Pikachu appears in your living room. He quickly assumes control of the TV, protesting noisily if you don't switch to something he likes to watch. He actually stays with you through the entirety of the game, which is really annoying. After a few days, the Magnemite who brings you the disks, loses it on the way. Prof. Oak asks you to locate and retrieve the disk, and this is the only occurance of anything resembling a storyline in the entire game. The disk contains the final episode of the Pichu & Pichu series, and it is of utter significance that it is found. Obviously, finding the disk isn't hard at all, only time-consuming and dull. When you find the disk you have in essence completed the game.
The people who came up with the idea for this game hopefully aren't in the video game business anymore. This isn't even recommendable for children or die-hard fans.
The only upside to this game (and probably the only reason it sells) is that you, upon completion of the main story, can trade Jirachi to your GBA game.
There is, however, a way to trick the game into running the story faster. You can go to the time and date settings on your GC and adjust the date one day ahead. That way, when you turn the game back on, a day will have passed. If you do this about seven times (I think the story spans over a week) you will eventually get to find the final disk of the Pichu & Pichu show. After that you can talk to Prof. Oak, Jirachi will appear and Bob's your uncle.