Iona Potapov is grieving over the death of his son. He tries to tell people what he is feeling and thinking, but nobody shows interest or pity for a forlorn old cab driver. The theme of the story is contained in the epigraph, which reads:
"To whom shall I tell my grief?"
It's a good question to which many of us would like to know the answer.
To Potapov it is a event of great importance. He thinks it is especially strange that a father should outlive his son. He might feel some relief if he were able to share his grief with another human being--but all the passengers he picks up are preoccupied with their own thoughts and plans. Finally Potapov decides to quit work for the night and heads back to the stables. Evidently he does not want to be alone, so he stay with his horse for a while after he has unhitched her. He begins talking to her and imagines that she can understand him. Perhaps the horse really can understand his feelings even if she cannot understand his words.
"That's how it is, old girl.... Kuzma Ionitch is gone.... He said good-by to me.... He went and died for no reason.... Now, suppose you had a little colt, and you were own mother to that little colt. ... And all at once that same little colt went and died.... You'd be sorry, wouldn't you?..."
The little mare munches, listens, and breathes on her master's hands. Iona is carried away and tells her all about it.
The irony in "Misery" is all contained in the fact that the simple old man imagines he can communicate his misery with a dumb animal. Irony is usually something that would be funny if it were not so painful or pathetic. Some people might laugh at him if they saw him. No doubt Potapov does get some temporary relief from talking to his horse, which seems to be the only friend he has left in the world.
In Macbeth, Malcolm tells Macduff, who has just learned of the slaughter of his wife and children:
Give sorrow words. The grief that does not speak
Whispers the o'erfraught heart and bids it break. IV.3