Simon Fish was an exiled Protestant reformer and author during Season One of The Tudors. He is mentioned admiringly by Thomas Cromwell in episode 1.09 and appears in the Season finale, having returned to England after the downfall of Cardinal Thomas Wolsey as King Henry's First Minister. He is the author of A Supplication for the Beggars, a famous essay denouncing the abuses of the Clergy and especially the Pope in living in luxury at the expenses of the common people; Fish referred to the Pope as "a vicious bloodsucker."
Fish and other prominent English Protestant thinkers were exiled to mainland Europe (mainly to Holland and the German States, where Protestantism had taken a stronger hold) by Cardinal Wolsey, a strong (though very corrupt) advocate of Catholicism. However, they continued to write and publish works condemning the abuses of the Papacy and the Clergy. Fish apparently kept contact with fellow Protestants Anne Boleyn and Thomas Cromwell, as Cromwell described him to Anne as "our mutual friend". Having heard of Henry's frustrated plans for an annulment to his marriage to Catherine of Aragon, and knowing Henry wished to marry Anne, Fish (via Cromwell) sent her a book by his fellow Protestant William Tyndale, The Obedience of the Christian Man. Tyndale's work not only denounced the power of the Papacy, it also made an argument for Kings operating as Head of the Church within their respective realms- thus giving Henry an incentive to support Protestantism and break with Rome.
In episode 1.10 "The Death of Wolsey", Fish returned to England, believing that, with Wolsey deposed, the attitude towards Protestants might be softening. However, he acted far too soon. Anne was not yet Queen, and Wolsey's replacement as Chancellor was Sir Thomas More, an even sterner and more unbending Catholic than Wolsey. Fish was promptly summoned to More's home, where More questioned him about other potential Protestants at court; however, Fish quickly realized the danger and remained silent. More then read passages of A Supplication for the Beggars, expressing his fury at the way Fish had desecrated the sacred nature of the Pope himself. Frightened but defiant, Fish claimed to be a true man of God, but More retorted that he was a heretic. Shortly afterwards, Fish was sentenced to be burned at the stake; More attended the execution and encouraged him to recant his heresy and escape his fate, but Fish responded by defiantly reciting Psalm 23 as he began to burn.
The actual Simon Fish was indeed arrested by Thomas More on returning to England and condemned as a heretic, but he was not one of the six people burned at the stake during More's administration; he died of plague while imprisoned in the Tower.