What is the story of the poem "Ode on a Grecian Urn?"

In this poem, the narrator has found an old urn (like a vase) that was made long ago by some Greek person.  The urn is decorated with two scenes -- one of lovers listening to musicians under some trees and one of a priest leading a heifer to be sacrificed on an altar.

So the narrator is looking at this urn and contemplating what it means.

First he thinks it would be great to be the lovers or the musicians because they never die.  Their love and their music live forever.

But then he starts to think about how the lovers will never actually touch and make love, the musicians will never be heard by actual ears. And the town where the people are coming from to the sacrifice -- it will always be empty and desolate.

So by the end of the poem he's kind of conflicted and he thinks that because we're mortal we have to find something other than beauty to rely on.