The poem describes a speaker who takes a boat out on a lake one summer evening and feels happy and confident as he admires the beautiful natural scenery. However, he suddenly sees a huge, dark mountain on the horizon which seems to tower over him and pursue him. Frightened, he turns back to shore and is left feeling alone and troubled, with the powerful image of the mountain lingering in his mind. The encounter changes his perspective on nature from something pleasant to something more immense and unsettling.