Burns's narrator addresses the mouse in this poem whose nest has been upended by a plough in November, just as winter is coming. He apologizes to the mouse, saying
I’m truly sorry Man’s dominion
Has broken Nature’s social union,
An’ justifies that ill opinion,
Which makes thee startle
This leads to the first serious theme of the poem, which is that humankind's dominion or rule of the earth is out of sync with the natural order, breaking the harmony of nature, and that this is wrong: the mouse is justified in its "ill opinion" of humans.
The poem continues by describing the suffering this breaking of "nature's social union" has caused the mouse. It has worked hard to build the nest that is now blown to the winds at a time of year when cold is coming, and there are no materials left to gather up for a new nest. Humankind's interference with the natural order is cruel. The narrator goes on to compare the mouse's fate with that of humans—the best-laid plans of mice and men both can go awry.
The narrator ends on the second serious theme of the poem, saying to the mouse that for all its suffering, it is blessed compared to him, because it lives only the present moment. Humans, in contrast, carry the weight of past memories and future fears with them. This conveys a bleak view of life, positing a world out of joint and full of pain.