While Metroid has always flirted with horror, Fusion goes as far as the series ever has, with some light body horror, a derelict space station (much creepier than being on a weird alien planet imo), and a heartless copy of yourself that is tracking you, destroying pathways you need to access, and generally being a pain in the ass. I don't want to put too fine a point on it - this is still a Metroid game. But that oppressive atmosphere, coupled with the game's mission-based structure - clearly designed for the short bursts of play a handheld game thrives at - that cuts down on needless wandering of one large area in favour of densely-packed smaller ones, and basic-but-functional character writing, makes Metroid Fusion a clear highlight of the series.