‘Night Sky’ imagines the life of that sweet older couple next door with a sci-fi twist

You know that pleasant older couple across the street, who mostly keep to themselves? What if they had a portal to another planet in their backyard? That’s the out-there premise of “Night Sky,” an Amazon series that combines science fiction and mystery with that rarest of commodities — a touching love story involving an old married couple — in an intriguing if plodding format.