But Mousie, thou art no thy-lane, In proving foresight may be vain: The best laid schemes o' Mice an' Men Gang aft agley, An' lea'e us nought but grief an' pain, For promis'd joy!
The stanza starts with the speaker telling the little mousie that she's not alone ("no thy lane") in showing that foresight, or looking ahead to plan for the future, can be in vain, or a pointless exercise.
And now come some of the most famous lines of poetry OF ALL TIME, so hold onto your hats:
The best-laid plans, whether you're a mouse or a man, often go awry.
The speaker connects mice and men through analogy in this line, just as he did earlier in lines 11-12.
So when the speaker says that plans going all to heck leave US nothing but grief and pain, instead of the joy we'd expected, he's talking about mice, men, and all mortal creatures, all over the world. The problem of having your plans get all screwed up, in other words, is universal.