A great science fiction premise could be that a technologically advanced adversary genetically engineers mirror life people to work as spies and saboteurs at wiping out the rest of the population so they can have the world for themselves, but they get rumbled by dogs specially trained to sniff them out or various other means that might surprise and delight the reader. I vaguely remember reading somewhere that the difference between the smell of an orange and a lemon is down to the chirality of a single molecule.