I don't think that ''every'' story can be summed up in one sentence (particularly with super confusing ones like 2001 or Derek Jarman's) nor that doing so sums up every subplot as well, but most stories do have one theme that's stronger than the others. Reason being because many stories are inspired by a single idea, such as "what if two teens from feuding families fell in love" or "what if a woman had to tell a thousand stories to stall her execution". Not all, but many.

With Lord of the Rings, I guess it'd be "Mighty evil can fall only if you make your rivals your friends." Whether those people you resent be a rebellious neighboring kingdom, a crafty thief creature, those haughty elves or uncouth dwarves, or even those hobbits you look down upon. That's my take, but I'm no Tolkein scholar.