The ghost of Brigadier Lethbridge-Stewart hangs around the final scenes of The Christmas Invasion. Back in 1970’s Spearhead from Space, he mentioned how humans had been sending probes deeper and deeper into space. “We’ve drawn attention to ourselves,” he said ominously, moustache twitching with deep portent.

David Tennant’s skinny, sassy Doctor has just seen off the slave trading Sycorax in the space of about 10 minutes, but still he’s cautious. He all but quotes his old friend. Though he’s got quite the gob, so he uses far more words to say it: “And the human race is drawing attention to itself. Every day you’re sending out probes and messages and signals. This planet’s so noisy. You’re getting noticed more and more.”

His words spook Harriet Jones, Prime Minister (Penelope Wilton). It’s been a bad day at the office. She’s just been through an alien invasion she was powerless to stop, she’s seen two men die in front of her and had the fate of half the world rest on her ability to negotiate her way out of an impossible situation.  She’s been in charge when the Doctor didn’t show up, and it’s terrified her.

So she takes a decision to blow up the alien threat rather than let them escape. In doing so, she’s channeling he Brigadier. He took the same pragmatic choice when he blew up the Silurians, again in 1970, in a desperate attempt to end a story which had already gone on for seven episodes. The difference then was we never got to see the Doctor confront his military friend about his act of murder, masquerading as defence.

Here, the Doctor forces Harriet to justify her choice. This she does, with hawkish pragmatism. “You said yourself, Doctor, they’d go back to the stars and tell others about the Earth. I’m sorry, Doctor, but you’re not here all the time… They died right in front of me while you were sleeping. In which case we have to defend ourselves.” The Doctor is, of course, disgusted. “But that’s murder,” he said in 1970 and so he repeats, “that was murder” in 2005. Apart from that, he doesn’t bother to try to counter her arguments. He just starts tossing around threats.

More of that later. But first, it’s interesting that writer Russell T Davies is specifically referencing those two stories from 1970 (three, if we note that trouble with aliens abducting a British space craft was core to The Ambassadors of Death). He even goes to the extent of quoting them, almost word for word. He’s reminding us of the time when the Doctor had an uncomfortable relationship with his Earthbound allies. And also of a time when a new Doctor made a barnstorming entrance, signalling a major shift in the tone and focus of the series. David Tennant’s Doctor signals as significant a progression for the series as Pertwee, colour and exile to Earth did.

Tennant’s Doctor is different to Pertwee’s though, in that he’s unafraid to meter out punishment if you cross him. When the Sycorax leader goes back on his word to leave Earth, and instead redoubles his attack, the Doctor has no hesitation in triggering the trap door which sends the bad guy plummeting to the ground. “No second chances,” he says grimly. That goes for Harriet too.

As his argument with her escalates, he warns her of the consequences of messing with him. And when she shows no remorse, he decides to bring down her government by whispering six words in the ear of right hand man, Alex (Adam Garcia, formerly a red hot tap dancer back in Australia. Mrs Spandrell was very keen on him.) It’s a handy trick. I wish he would fall to Earth now and perform that same feat in the USA.

Anyway, the point is that this Doctor is not a man to cross.In some ways, that rift with Harriet marks the tenth Doctor out as political; he’s against pre-emptive military action. Or maybe it’s simpler than that – he just against the sneaky tactics of clobbering someone from behind.

Either way, he’s unafraid to lose friends when he thinks they’ve done the wrong thing. Later he watches Harriet on TV, flustered by questions about her health, engulfed in the PR storm he’s just conjured up with a six word magic spell. He stands there in his new glasses and paper Christmas hat and watches his former friend’s world collapse around her, and he’s unmoved.

This will of steel is something he has in common with his predecessor, who watched dispassionately as Cassandra burst apart and who dumped failed companion Adam back to Earth with window in his forehead. But then unlike the ninth Doctor, he does domestic. He has Christmas dinner with Jackie (Camille Coduri) and Mickey (Noel Clarke), something the last him flatly refused to do. Indeed his whole attitude to Jackie and Mickey has softened. He physically embraces them – again something he previously wouldn’t have had a bar of. So although he’s just as uncompromising as Dr 9, he’s a far more accessible and relaxed with his human buddies.

There’s one last moment that underlines Doctor Tennant’s refusal to let his human compadres take the easy way out. At the story’s end, when he goes to grab Rose’s hand to run off together for further adventures, she shies away a little because it means holding the hand he recently grew back. “That hand still gives me the creeps,” she says. But he doesn’t offer her the other one. He insists she takes the one that freaks her out. It’s a tiny little moment, but it just reinforces that this Doctor doesn’t let you off easy.

One last thing to note. This is the story which starts to develop Mickey and Jackie as characters, beyond being handbrakes on Rose’s TARDIS adventuring. Mickey gets his first heroic moment when he outmanoeuvres the robot Santas and Jackie plays both caring matriarch and comic relief (I particularly love her reminding Mickey to note down how much internet he uses, even though only moments ago they were nearly killed by a rampaging Christmas tree). They are, at last, the Doctor’s allies, Earth-bound but ready to help out when needed. Pertwee had his UNIT family. Tennant has the Tyler family. The Brigadier would be pleased.

LINK TO The Power of Kroll: both were originally broadcast on/around Christmas time.

NEXT TIME… The walls need sponging and there’s a sinister puddle. We’ll take care of it and The Caretaker too.