Lego Star Wars: The Video Game takes you through the latest three Star Wars movies. The game is broken up into three episodes (a bit obvious) and each episode is broken up into five or six chapters. These chapters graze over the major events of each movie. You will participate in the retaking of Theed, the Colosseum battle on Geonosis and the space fight over Coruscant, as well as events like fighting Count Dooku, Darth Maul and of course the Mustafar fight between Obi-Wan and Anakin.
Lego Star Wars has two modes of play, Story Mode and Free Play Mode. In Story Mode, you play through the events of the movies, collect studs (currency) and Lego Canisters to unlock characters and extra levels. After you beat a chapter in Story Mode, you can go back to it in Free Play Mode. It is here where you can actually get to all of the collectibles since you are allowed to switch between various unlocked characters at will. Using each character's special abilities (double jumping, opening doors, hovering, etc.) will let you get everything you need in order to be done with the level completely.
So which characters can you unlock? Good guys like Qui-Gon Jin, Anakin, Obi-Wan, Padme, R2-D2, Mace Windu and Yoda get unlocked once you beat the first mission that lets you control them. In order to unlock the enemies (Battle Droids, Sith, General Grievous) you have to beat them in a level and then use your studs to buy the character in the game's hub area.
The GameCube version of Lego Star Wars plays just like the other versions. Everything from the missions to the characters are exactly the same. The only noticeable difference is the fact that there is no autosave feature. In the other versions, once you completed a mission or unlocked something in the hub area, the game will automatically overwrite your current game. But in the GC version, every single time it would have autosaved, it asks you which file you want to write to and if you want to overwrite and then continue. Had I not known of the autosave feature found in the other versions, this might not have been bad. But since I knew that the PS2, Xbox and PC versions of the game did have this feature -- I found this to be one of those pointless changes that make no sense.