The great wizard Merlin is usually considered to be the architect of King Arthur's legendary reign. He has been portrayed since the early Middle Ages as both brilliant and flawed, good but selfish, powerful and yet prone to human weaknesses. He was certainly eccentric, but was he a bit mad? The renowned enchanter's greatest gift was his ability to see into the future and prophesy, yet he was concerned more about his own desires and vanity than the fate of others or even the truth.
Knight for a Day reveals Merlin as a tired and slightly absent-minded old man seeking to avert an imminent disaster. He recklessly interferes with the continuum of time in a desperate experiment to save Camelot from the forces of evil and to preserve his own reputation and legacy.
Merlin instructs Henry how he can initiate a spell to return home, telling him he's also a wizard since he already invoked the magic of the stone and has a knowledge of the future. He promises Henry a reward beyond his wildest dreams if he can succeed on his secret mission for King Arthur; but it's an almost impossible task that will turn into a race against time and could strand Henry in the past for ever.