Sci-fi as a genre is very difficult to perfect, but some terrific movies have come close, only to be held back by one or two flaws. Sci-fi deals with big ideas on an epic scale, and it's not easy to come up with a concept that connects with an audience. Even if a sci-fi movie does find an intriguing premise, translating this into a compelling narrative can be a tough challenge. Sci-fi relies on tropes like any other genre, but it also requires a lot more big-picture thinking, and this can cause some writers and directors to stumble. Even if a sci-fi movie isn't perfect, it can still be wildly entertaining and full of mind-bending concepts.