If the original Metroid was a coarse prototype of Super Metroid, Zero Mission is a functional reiteration of the formula, the next best thing when it comes to sequence-breakable 2D Metroid (nor do I think it reaches the heights of Fusion, but that's beside the point) and a nifty game at that. The ever so memorable Zero Suit section spices things up and lends the game an additional sense of identity, the framework it was working with already being fairly sound.