On Earth, Sylvia is thrilled to see the Daleks pulling back; she doesn't know that they've been recalled to shield in the Crucible, although Wilf is wary, especially as Donna is still missing.

Jack burst through a ventilation duct to find, by his own description, "Mickey Mouse."  Mickey calls him "Captain Cheesecake" in return before they launch into a happy, slashy hug.  "That's beefcake" Jack tells him. 

"That's enough hugging," Mickey replies.

Jack turns to salute Sarah Jane, who pulls a pendant out of her pocket, one of the many tchotckies she got from the Veren soothsayer.  This one is a Warp Star, which is essentially a portable explosion.  Sarah Jane hands it to Jack.

Martha has her two contacts; it only takes three to use the key.  "Do we do it?" one of her counterpart asks.  "UNIT orders..."

But Martha cites a higher authority.  "There's one more thing the Doctor would do."

On the TARDIS, human!Doctor is working out a way to subvert the gun, by locking it onto Davros.  As all these Daleks except Caan were grown from Davros, it will turn the reality bomb "into the biggest backfire in history."

Martha calls in, and the Supreme Dalek patches the transmission down to Davros and the Doctor, who asks to be allowed to talk to her.  In his corner, Caan burbles and flails, giggling about how "the Children of Time will gather and one of them will die."

"Stop saying that!" the Doctor snaps.  My sentiments exactly.  That's gotten a little old.

Although Martha greets the Doctor, it's Davros who takes the lead, asking her to state her purpose.  The Osterhagen key will detonate 25 nuclear warheads scattered across the planet, blowing Earth to bits.  The Doctor is understandably appalled, but Martha is firm.  The key is to be used if the human race is suffering without hope; a mercy killing for it."

"That's never an option!" the Doctor snaps.

Martha snaps right back at him.  "Don't argue with me!  I reckon the Daleks need these 27 planets."  She raises her key.  "What happens if it becomes 26?"

"She's good," Rose says admiringly, a 180 from her attitude in Stolen Earth.  Martha, on introduction, is stunned that the Doctor found Rose.  But a second transmission interrupts the healing of the shipper war.

"Captain Jack Harkness, calling all Dalek boys and girls," Jack carols.   It's hard to say if Rose is more shocked to see him still alive or to see her mother behind him.  Mickey and Sarah Jane are there as well.

Jack has his own stop-or-die proposal; he's wired the Warp Star into the Crucible; if they do anything, he'll blow it out of space.

"Where did you get a Warp Star?" the Doctor asks, horrified.

"From me," Sarah Jane says.  She goes on about how there's no other alternative, but Davros is fascinated to see his old foe.  (Sales of "Genesis of the Daleks" must really have boomed after this episode.)

Sarah Jane isn't frightened now.  "Remember?" she spits back at a delighted Davros.  "I've learned how to fight since then.  You let go of the Doctor or this Warp Star, it gets opened."

Rose is thrilled to see her people fighting back, but the Doctor is silent and horrified. Davros is jubilant.  This is the soul of the Doctor - "who abhors violence and never carries a gun... You take ordinary people and fashion them into weapons.  Behold your Children of Time transformed into murderers.  I made the Daleks.  You made this.
"

It's an incredibly powerful moment, and a theme which I wish they'd dwelled on for a bit longer - but at the same time, it's hardly as though the Doctor invented the Osterhagen key or that the human race, when left to itself, only gathers flowers and knits toilet paper cozies in universal harmony.

Davros twists the knife, saying that he has already seen the Children of Time sacrifice one of their own. It's the first that the Doctor has heard of Harriet Jones' fate.  "How many have died in your name?" Davros asks, and it's a long line of flashbacks to assorted characters who have fallen.  Good thing they stuck to New Who; if they'd done the whole series it would have taken a full day just to air this segment.  They also only show the people who fell fighting for the Doctor and not the ones he killed, which is an even longer list.

David, always at his best in the silent scenes, radiates anger and horror as Davros gloats, "The Doctor.  The one who always runs and never looks back because he dare not out of shame.  This is my final victory.  I have shown you yourself."

Martha braces to use the key.  Jack braces to break the warp star.  But before they can do anything, the Supreme Dalek teleports them to the Doctor and Davros.

At the Doctor's urging, they kneel and surrender.  "Mum, I told you not to," Rose hisses. 

"I couldn't leave you," Jackie replies.  It's great to see Jackie, but hasn't she left behind an infant to go haring after her grown daughter?  No wonder Rose thinks love means never getting more than 20 feet away from  whoever you're with.

"DETONATE THE REALITY BOMB!" Davros howls.

With much CGI and swelling music, something that looks far too much like the Death Star starts to gear up, while Davros shrieks with insane laughter about how nothing and no one can stop him.

Except for human!Doctor and Donna, who are just about to pull a Bad Wolf and vworp their way in to save the day.  It's Caan giggling as the human!Doctor comes running out of the TARDIS with his reversal gun...

Only to be stopped by a bolt to the heart from Davros.  Good plan, poor execution, human!Doctor.  Donna launches out and grabs the gun, to be felled in her turn.  The Daleks casually exterminate the gun into bits.

In a moment stolen straight from reality shows, the tension is randomly held so that Davros can redeliver exposition about the state everyone is in, the music can get louder, and a little more CGI can be thrown in. It's nothing but padding to draw out the moment before the Reality Bomb fires.

The countdown finally reaches "Fire!" - and the power fails.  Donna, the only one not in a spotlight cell and now standing in the middle of the lashup of equipment human!Doctor built, technobabbles about how the feedback loop has been disabled.  Davros tries to zap her again; she throws a lever and his glove backfires.  The Daleks wheel on her; with some rapid playing of levers, their guns refuse to function.  She technobabbles what she's done so fast I can't even follow the words, but the explanation of what has happened to her - two-way biological metacrisis, with her half being activated by the energy bolt, is delivered at a more understandable pace.

Well, more understandable if you're not that demanding in the logic department. 

"DoctorDonna!" the original Doctor says, shouting out to Planet of the Ood.  A delighted Donna drops all the holding cells and orders the "skinny boys in suits" to get to work.  Both Doctors launch themselves back to the TARDIS, but it's Donna who has made the Daleks dance and twirl out of control.  (I hope the guys inside were having fun with that scene!)

Donna couldn't be more pleased with herself.  She's figuring things out faster than the Doctors "because you two are just Time Lords, dumbos!"  It's the bit of human that has given DoctorDonna new perspective that the Doctor(s) doesn't have.