The main image of "The Mouse on the Bridge" is encapsulated in the title. The poet is crossing a bridge over the Saskatchewan River when she sees a mouse "scurrying in front" of her.

The rest of the poem subjects this single, central image to various different thoughts and points of view. The poet wonders what the mouse can possibly be doing there, amidst the noise and pollution. She recalls other mice that she has seen in the countryside, in fields and farmhouses. She associates mice with rural life and with Robert Burns's famous poem "To a Mouse" but not "with city traffic." She sees the mouse as being out of his element and wonders "how long his little nerves will take it."

Having worried about the safety of the mouse, the poet reflects that "Mice have never had easy lives" and thinks that a nest in this environment is probably as good as one anywhere else, no more incongruous and perilous than her own living situation in a "teetering" high-rise tower.

The poet ends by contrasting the mouses's freedom with the cell-like nature of much of humanity's habitat, banks, office buildings and classrooms. These well-stocked "cubical cages" keep people in captivity, as people sometimes keep mice in cages. The image of the mouse running across a bridge, therefore, begins as an incongruous intrusion of the natural world into the city and ends by causing the poet to meditate on the unnatural nature of her own living situation.