The masked Romeo, Benvolio, and Mercutio are on their way to the Capulet ball. As they proceed, Mercutio mocks Romeo's romantic sighings with a series of ribbald puns. Romeo refuses to join in this erotic horseplay, explaining that in a dream he has been warned not to attend the feast. At this point, Mercutio delivers a long, progressively darker monologue about Mab, who visits people in their dreams. Shakespeare's audience would have been aware of the contradictory allusions in the monologue, recognizing, on the one hand, the quasi-mythological queen of the fairies, and on the other, the reference in the words "quaen" and "mab" - synonyms for prostitute. This double entendre is well illustrated in the development of the monologue. As Mercutio begins, Mab is a figure even children would thrill to: She drives into the world of dreamers in a walnut carriage drawn by insect horses. But by its conclusion, which Romeo himself effects, saying "Thou talks't of nothing," Mab is a "hag" - not only a whore, but a whoremonger who teaches "maids [to] lie on their backs, ...making them women of good carriage." Along the way, we are treated to the dreams that come true for the depraved and violent aspirations of the dreamers: lovers dream of lust; lawyers dream of exorbitant profits; and soldiers dream of “cutting foreign throats” (1.4.83)
The play is also full of dreams - Romeo and Juliet dream of romantic love uniting two implacably hostile houses; Tybalt dreams of aristocratic rectitude; and Friar Lawrence dreams of a Verona at peace. Mercutio's speech, taking place as it does just prior to Romeo's first encounter with Juliet, and his tragic destiny, offers a kind of 'deconstruction' of the phantasmagorical drama that is Romeo and Juliet. Although Mercutio's Queen Mab speech does not demolish these dreams - after all, Montague and Capulet pledge to honour the tragic deaths of the lovers - still it serves as a kind of counterpoint to them.