The plot that Keats develops as he muses in this lyric poem is intertwined with the suggestion of his feelings and themes. The central feelings the Grecian urn inspires in Keats are wonder, "dost tease us out of thought / As doth eternity," and conflicted envy, "and for ever young; / All breathing human passion far [greater than]." The themes that dominate are the awareness of the lifelessness in eternal beauty, "Cold Pastoral! / ... / Thou shalt remain," and the transience of human experience, "old age shall this generation waste."
The first and second stanzas describe a wedding and are connected by the urn's wedding theme: "unravished bride." In the first stanza he speaks of the bride as one who, caught in woods of timelessness as a "Sylvan historian," can express the meaning and beauty of the urn better than his poetry, his "rhyme," can. In the second stanza Keats speaks of "melodies" of "pipes and timbrels" and of the groom, the "Bold lover," who, frozen in time, can never consummate his wedding with a kiss nor his marriage with a bridal night: "Bold Lover, never, never canst thou kiss." Keats consoles him with the knowledge that his love and her beauty can never fade.
When speaking of the music of the piper, “Heard melodies are sweet, but those unheard/ Are sweeter," Keats' thoughts relate to the spirit mentioned, "Pipe to the spirit." Real tunes are flawed. Those piped on the urn to the "spirit" are without flaw. Yet Keats sees that their flawlessness is a flaw in itself since the melodies have "no tone," no music. Keats also foreshadows in the line "never, never canst thou kiss" the idea of the "Cold Pastoral!" of the urn's "marble" that is flawed by being without warm, flowing life, like the music is flawed for being without music.
The second, third and fourth stanzas develop the theme of the flawed nature of lifelessness in beauty: each marble, immobilized scene on the urn is flawed while flawless. The town following the priest of Hymen out to the marriage with the wedding's sacrificial young cow is frozen in flawless beauty, but the town is flawed by being desolate, without any who will ever return. The "more happy, happy love" is "for ever warm" "panting" and "young" and it is "far above" [far greater than] all "breathing human passion" since it is flawed by being fragile and changeable. The bride, groom and piper are frozen in purity, beauty and musicality, yet the tune is toneless, the kiss is undelivered, and the bride is ever a virgin. The love shown on the urn is unlike human love that suffers emotional, spiritual and physical depletion: "a heart high-sorrowful and cloy'd, / A burning forehead, and a parching tongue."
Turning to the theme of Truth and Beauty, Keats ends his narrative of musings by addressing the urn--decorated all over with forest, men and maidens--and accusing it of escaping understanding through contemplation in just the same way that eternity escapes understanding through contemplation: neither the urn nor eternity can be known through contemplation and musings. He ends by recording the message the urn gives in reply to his accusation: "Beauty is Truth, Truth is Beauty." The paraphrase helps focus what Keats means:
PARAPHRASE: The urn answers back, "All you need to know is that Beauty [the urn] is eternal, outlasting all other things after they are dead and gone, and Truth shows itself [love, nature, religion, marriage] in the Beauty that survives. This is all you can know; musing and contemplation can gain you no more knowledge than this."