Here we go: Mariana is a young lady who has been recently jilted by her fiancé. She gets dumped when her father's dowry dries up, actually. Her (ex-) boyfriend = not a nice guy. To recover from her heartbreak, she holes up in a farmhouse that is surrounded by a moat—just like a princess from a fairytale.
Just one problem: he never comes. Pretty much that's what happens in the poem, too—and not much else, if you think about it. Tennyson isn't re-writing Mariana's story, though, or adding a happy (or sad) ending. He's just lingering with her, in the farmhouse, for a little, and letting us see what it's like to live with despair and sorrow. Um, thanks?