By the time we get from the first picture to the start of the story proper, the Doctor and Tegan have been captured and put in a cell. The Doctor is clearly in an irascible mood, since he spends the first page chiding Tegan for believing the history she was taught at school, and then stresses the importance of learning history.
We get one piece of exposition, that they are "on earth. In Britain to be exact" before they are taken to see King Arthur.
King Arthur questions them about where they come from, and the Doctor immediately tells him about the Tardis. King Arthur has heard the word "Tardis" before but, in an attempt to introduce some suspense to the story, he can't immediately remember where from.
King Arthur tells them about the Merlin:
"The Merlin has no name. The Merlin is my necromancer, my adviser, my bard - he serves a variety of uses, and is held in very high regard by me and my court . . . Ah!"
This reminds King Arthur where he has heard of the Tardis before.
"When he first appeared at Camelot, just after my coronation and the death of Blaise, the old Merlin, he said his ship, his vessel - call it what you will - was called Tardis."
Tegan gasped. "The Master!" she whispered in horror.
The Doctor nodded grimly.
This is a bit of a leap for Tegan to make, although it may be somewhat reasonable if the Master was the only other Time Lord she had met or heard of by this point, but it is even more extraordinary for the Doctor to assume it must be the Master. Although given how often the Master shows up compared to any other renegade Tine Lord, the balance of probabilities suggest he'll turn out to be correct.
The Merlin always dresses for the occasion.
Regardless, the Doctor decides to malign the Merlin.
"I have known him for a long time, and he is an evil man. He will aim to destroy you and your king-dom if he can, to change history to suit himself.
After some further questioning of King Arthur the Doctor deduces that the Master's plan is to save Mordred so that he will kill King Arthur at the battle of Camlan.
"I have met him before, in other places. Every time, he was attempting to bring about death and evil. I alone could stop him. I am afraid that in this case, he has already done what he set out to do - he has protected Mordred, your son, from certain death, so that the child can grow. I can do nothing about that now. The child will be well hidden. Nobody could find him.
It sounds like the Master's plan in this case is to not change history, the fiend! The Doctor concludes that the Master is also responsible for the Saxons currently raiding Britain. As King Arthur sends for the Merlin, the Doctor continues his denunciation.
"He is not a Merlin. He is not a good man; he is not even human."
"Forgive me, this may be hard to explain. The Master, as I know him, and I are what is known as Time Lords. We have travelled to many different planets, even to different galaxies, and to many different times.
The Doctor is quite happy to tell King Arthur about his future, seemingly unworried that this might have any unintended consequences.
This artist is really not good at drawing Tegan's face, and I'm not sure that's Davo playing the Doctor in this story...
At this point the Master enters and saves the Doctor from an embarrassing defamation lawsuit. He doesn't deny knowing the Doctor and Tegan, but he does make an attempt at claiming not to be evil.
My lord King, have you considered - forgive my presumption - but has it occurred to you to think that these strangers might be the enemy? You have seen the services I have done you and your court. I have always done whatever lay in my power. And what do you know of this stranger here? What proof do you have of his goodness?"
On consideration, I think it's Tim Brooke-Taylor playing the Doctor today, but I'm not completely sure.
King Arthur doesn't need proof (being High King has its advantages), he decides the Master is guilty based on his having failed to prevent some bad things happening to the kingdom during his time as the Merlin (also, and I'm sure this is coincidence, Arthur's time as the King). He orders the Master to be taken to his room and guarded there. Having been rumbled, the Master - in accordance with tradition - drops any pretence and does the evil voice.
The Master smiled ironically at the Doctor, then bowed mockingly to Arthur. "Foolish mortal," he said, almost sorrowfully, "I shall, as you wish, go to my room."
After a few moments the Doctor and Tegan realise that the Master's Tardis will be in his room, but by the time they get there he has gone. Having prevented the Master's diabolical plan to ensure that the history of King Arthur remained unchanged, the Doctor tries to make history unchanged even more.
We must build the Arthurian legend."
"And how do we do that? the King asked wearily. "I have no trustworthy advisers now, except my generals and captains and knights."
So he has some then? Confused cat is confused. The story ends with the Doctor asking:
"Tell me, King Arthur," he asked, "have you such a thing as a large, round table?"
This is a terrible story. If Night Flight to Nowhere is comparable to Time Flight, then The Greation of Gamelot resembles The King's Demons. And while that TV story isn't great, at least the Master is a proper baddy in it, with an evil plan for the Doctor to stop and everything.
The entirety of this seven-page story is built around two scenes of dialogue, in the first of which the Doctor just asserts that the Master is involved and must be up to evil, and in the second he foils the Master's plan - which, while not nice, is by all appearances him attempting to preserve history - because King Arthur believes him on the basis of no evidence.
In short, the Doctor is in the right only because he's the Doctor, and the Master is in the wrong only because he's the Master. This feels very lazy - the story spends so much time on exposition that it could not even put in a bit of the Master doing something actually evil, or provide a sensible reason for Arthur to trust the Doctor instead of his adviser. (King Arthur isn't an evil king scapegoating his adviser for the failings of his own rule only because he's King Arthur.)
Night Flight to Nowhere seemed unusual in featuring the Master after so many stories without any TV baddys. For him to return only three stories later, and in such a bad story at that, it already seems to me that he's overused. In that respect this does a good job of capturing the feel of the TV series of the era around when it was written.