The narrator is an old, weathered, and wizened seaman who meets "three gallants" on their way to a wedding feast. He is able to detain one of them, and he proceeds to relate to this young man his tale of spiritual mysteries that are both wonderful and terrible. These mysteries that the Mariner has learned are best understood intuitively and emotionally rather than intellectually.

The purpose of the Mariner's strange tale is much like that of a parable since the Mariner preaches the lesson:

He prayeth best, who loveth best
All things great and small;
For the dear God who loveth us,
He made and loveth all. (615-618)

The Mariner's tribulations all began after he demonstrated no respect for one of God's creatures, the albatross, when he killed it with his crossbow. Certainly, his lack of respect for the spiritual world cost the sailor dearly--he tells the wedding guest, "I had done a hellish thing"--because the other sailors forced him to wear the dead bird around his neck like a cross when the winds stopped and they could no longer travel.

Further, the old Mariner states that after his killing of the albatross, a spirit, one of the inhabitants of the planet that is neither a departed soul or an angel, follows the ship, "plaguing" them. As his shipmates die, each "cursed me with his eye." The sailor can barely stand the staring eyes as the ship yet floats for the biblical "seven days and seven nights." 

Finally, one night the moon shone and the Mariner saw strange sights and "commotion" in the sky and on earth. Somehow the bodies of the dead crew are "inspired" and the ship begins to move. The Mariner credits "angelic spirits." Also, when he notices in the sea great water snakes who glitter blue, green, and black in the moonlight, he blesses them and the albatross falls from his neck; his life is redeemed.

The narrator is a hoary old seaman identified only as the 'Ancient Mariner'. He stops a man on his way to a wedding party to relate his lengthy tale of woe. This man is known only as the 'Wedding Guest'. The background to the whole scene seems somewhat random. The Mariner, we are told, simply 'detaineth' the Wedding Guest, for no particular reason, and begins to recount his frightening adventures at sea when he needlessly shot an albatross, thus bringing down retribution on his ship for wantonly destroying one of nature's creatures.

The overall frame of the poem, then, is very thinly sketched; the focus is entirely on the marvellous narrative of the Ancient Mariner, with its sublime and supernatural scenes and events. There is a suggestion of a moral of sorts, when at the end, the Mariner cautions the Wedding Guest to respect all of life and nature: 

Farewell, farewell! but this I tell

To thee, thou Wedding Guest!

He prayeth well, who loveth well

Both man and bird and beast.