The speaker has vague, anxious thoughts about his future as he walks on the moors one day, young and in good health. As he puts it, a "dim sadness" overtakes him, and he worries
there may come another day to me—
Solitude, pain of heart, distress, and poverty.
As chance or fate would have it, he stumbles on a very old man, the oldest he has ever seen, bent over, poor, and frail. The man seems to embody in the flesh all that the speaker fears about his own future.
The speaker talks to the man, who he finds out is a leech gatherer. The man is very poor and wanders from place to place. He tells the speaker that the supply of leeches to gather and sell is much less than it once was, making his hard work that much harder than it once was.
The hard life of the aged leech gatherer troubles the speaker at the end of the poem. He thinks:
The old Man's shape, and speech—all troubled me:
In my mind's eye I seemed to see him pace
About the weary moors continually,
Wandering about alone and silently.
However, as the leech gather continues to speak with him "cheerfully" and with kindness, the speaker is impressed and humbled by the old man's "firm … mind." He determines to remember this undaunted man and take inspiration from him.
The poem illustrates many of the key features of Wordsworth's Romanticism. It celebrates the character of a very humble figure, the kind of person usually overlooked in the poetry of his time. It takes place in the natural world, and it focuses on the importance of memory to guide and sustain us.