If there is a message in "The Miller's Tale" regarding marriage, it would appear to be that a significant age gap between husband and wife can often cause serious problems. The comely Alison is much younger than her husband John, and this makes her especially susceptible to the devious wiles of their smooth-talking lodger, an Oxford student by the name of Nicholas.
Alison is hardly what one would call a woman of the world. Despite being the object of desire for at least two men, she seems somewhat naive. Perhaps this partly explains her marriage to a much older man; like many women of her time, she seeks protection from the big old world outside. She wants some measure of security, the kind that John, for all his obvious faults, is able to provide.
But in choosing security, Alison has ended up with a man who cannot provide her with much in the way of attention. Nicholas senses this, and when John's away one day, he makes his move, and so his tawdry affair with Alison begins.
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