This story in a nutshell: We've only got 42 minutes to save the ship...
Mockney Dude: The Tenth Doctor immediately takes charge of the situation on the Pentalion, it's a role he is very accustomed to fulfilling when there is danger afoot (and why it was so effective when he failed so spectacularly to succeed in Midnight). 42 is almost a dry run for series four's superb chiller, showing the Doctor at his most confident (striding about, tackling several dangers at once, being witty and clever and working out precisely what has been happening to make the sun so angry) but also at his weakest too (once he is possessed by said sun and tortured horribly). It's an acting tour de force for David Tennant who is so in stride at this point he's got swagger. He keeps all the characters focussed on tackling the immediate danger and not letting their personal feelings get in the way. There is time to get in touch with your emotions when your not going to be roasted alive. Only the Doctor would be insane enough to cling aboard the skin of a ship that is so close to a roasting sun and look into its heart. Tennant's hysterical, terrified turn as the Doctor being consumed from within by a violently angry sentient sun that has had its heart torn free is something to witness. It's quite an admission from the Doctor to admit to being this scared and the way he growls that he could kill them all is genuinely frightening. Chibnall might not be pushing himself in narrative terms but he takes the Doctor to a very dark place and allows Martha to take control, both intriguing innovations. For a moment at the conclusion he is quietly affected by how close to the edge he was brought in this story before snapping back into his usual persona.
Medical Student: This is technically Martha's first outing as a fully fledged, paid up member of the TARDIS crew and given Human Nature sees her relationship with the Doctor played very different and they barely feature in Blink it is the only time she gets to just land somewhere and have an adventure in this role (Utopia counts too but that is the opening instalment to her exit story). Martha is wonderful in 42, it's a great story to show what she can bring to the series as a companion. Apparently simply being the Doctor's assistant is something of a dirty word these days but I don't see anything wrong with having a strong female supporting character that isn't trying to usurp the titular character of this show on a frequent basis. Martha gets to be funny (her pained reaction to how long her mum takes to switch on her laptop in a life and death scenario), flirty (her tactile relationship with Riley) and brings with her oodles of energy and charisma. All without having to shove the Doctor to one side and claim the show as her own. Martha's panic in the escape pod is palpable, Agyeman really going for it and giving a fearless performance. You get a real sense of the Doctor and Martha belonging together in a way that you have before when they reach out to each other across the vacuum of space. The fact that she can't here his repeated cries of 'I'll save you' is very touching. She realises that if she dies in the future that her family will never know what happened to her, she will have just disappeared. In a lump in the throat moment Martha tries to explain this to her mother (who is already suspicious that her daughter is in danger) without telling her anything tangible. With the Doctor in such pain at the climax, Martha gets the opportunity to save his life for a change and try and salvage the situation. She has more than earned her stripes at the end of this adventure, giddy at the thought of being given the key of the most wonderful craft in the universe.
Sparkling Dialogue: 'Well done, very hot.'
* The extended effects shot along the hull of the Pentalion as it is helplessly dragged into the corona of the sun convinces the viewer of the danger that the Doctor and Martha have found themselves in. The immediate peril means that the story gets its claws into you almost instantly and never lets go until the climax. Telling a story in real time across 42 minutes brings with it a lot of problems, not least not being able to take any narrative short cuts - you have to experience every action that the characters go through in this dramatic three quarters of an hour. But it also means that time is of the essence and there is always a palpable sense of danger. I think Chibnall and Harper pull of this immediacy with some skill, maintaining an incredible pace and stifling atmosphere.
* On paper the idea of Martha having to answer a pub quiz to prevent a spaceship from bring roasted alive and phoning home to her mother in another galaxy and time period to google the answers is absurd but on screen it transforms into something funny, touching and ominous. It's wonderful to have more scenes between Martha and Francine because they are needed to show just how much her mother cares for her (all she has offered her so far is scathing disapproval), it's amusing to hear Martha make up excuses for why she needs the answers and what the death rattling screams are in the background and the season arc is brought into sharp focus when we realise that the calls are being monitored for the mysterious Mister Saxon. Bravo on taking a potentially ropey idea and pulling it off with so much confidence that it becomes one of the most entertaining aspects of the story.
The Bad: You could complain that the supporting characters (McDonnell, Vashtee and Riley aside) are little more than canon fodder but there is nothing wrong with that in a story that has so much to do and so little time to do it in. Enough of the guest cast are given back story and the rest are there to die in spectacular ways (people should better than to say 'kill me now' in a Doctor Who story) and show how dangerous this creature is. They perform that function admirably. Michelle Collins does everything that story requires off her and it isn't what you could call a bad performance (in the Jenny Laird meaning of the word bad) but I could think of tons of stronger actresses who would have been able to have play the tough space captain with a heart. She's okay but the character thread would have had more impact had it been brought to life by an actress with more chutzpah.