If there is actually a plot that runs through an entire album (not just thematic similarities from song to song), and the plot of a science fiction or fantasy nature, then the entire project should be on topic. In this case, the music becomes just another medium for storytelling within the science fiction or fantasy genres. However, true storytelling albums of this type of not very common.
Sometimes, there will be individual songs with plots, but nothing tying the entire disc together other than thematic elements. The songs that, on their own, tell science fiction and fantasy stories are on topic; also on topic are questions about the genre themes that may tie the otherwise unrelated songs together. However, specifics about songs that are not part involved in the storytelling aspect are not on topic. For example, David Bowie released a number of science fiction concept albums, but most of the songs do not have actual plots. A few of the songs do actually tell science fiction stories—most famously "Space Oddity." It's not a very detailed story, but it is certainly a story, so the entirety of that song is on topic. For Bowie's non-narrative songs (such as "A Small Plot of Land"), questions about the science fiction references in the lyrics contain are topical, but questions about the purely musical elements are not.
If there are simply bits of science fiction or fantasy imagery overlaid onto an otherwise basically musical number, questions about the science fiction elements are on topic, but the remainder is not. For example Parliament Funkadelic performed a stadium tour that featured "the Mothership" landing on stage. Questions about the ship itself (its role in the show, the nature of the prop, etc.) are on topic, but most of the performance by Bootsy Collins and the rest of P Funk, not really being related to the spacecraft, are not on topic.