When we last left the Doctor, things were looking dire-The Master had conquered Earth, and he’d been reduced to a helpless, elderly state. However, Martha was able to escape.
A year has passed. The Master has fully weaponized Earth, intending to wage war with various alien species (So he can then conquer the whole universe). Martha, in the meantime, has been travelling the world, telling the story of the Doctor, and rumored to be assembling an anti-time lord weapon to take the Master out. She also captures a Toclafane, learning their dreadful secret-they’re actually the remnants of the future human race, who didn’t really find “utopia”, but became degenerate sphere-monsters instead.
Eventually, the Master captures Martha, and destroys her “weapon”. However, Martha has another trick up her sleeve-the stories she told of the Doctor. Eventually making the populace of Earth chant the Doctor’s name, reversing the Master’s hypnotic control, and causing enough energy to flow into the Doctor, reversing his aging and giving him enough power to disarm the Master.
This gives Jack enough time to disable the TARDIS’s Paradox machine modifications, undoing all the damage from the last year (except on the helicarrier itself). This causes the Toclafane to vanish as well.
The Master-now captured and powerless due to the rebellion and loss of the Toclafane-stands before the Doctor, Jack, and Martha’s family. The Doctor wishes to keep the Master prisoner inside the TARDIS, although Martha’s family would rather kill him for his crimes.
However, it’s Lucy Saxon who finally snaps and shoots him.
As he lies dying from the shot, the Doctor holds him and begs him to regenerate, as they’re the only two Time lords left-and despite all this chaos, the Doctor forgives his fellow Time Lord. The Master refuses.
The Doctor sheds tears as his “frenemy” dies.
As things return to normal around Earth-as all the chaos never “really happened”-with only the Doctor, Jack, Martha, her family and others aboard the helicarrier knowing the truth…The Doctor cremates the Master. However, as his body burns, a ring falls from it, and picked up by a female hand…
Meanwhile, The Doctor, Jack and Martha say their goodbyes. Jack decides to return to his “Torchwood” team, although he hints to the Doctor and Martha that, although his immortality can’t be fixed, he can still age, and may become the Face of Boe that warned the Doctor about the Master on New Earth-leaving the two stunned.
Meanwhile, Martha decides to leave too, to take care of her family, who’ve been through a lot-and also because she’s accepted that the Doctor is not interested in her romantically. Unlike Rose though, she’ll keep in touch.
As the Doctor prepares to take off, he receives an unexpected visitor: Himself.
Specifically, it’s the fifth Doctor (Peter Davison). The Tenth Doctor is actually delighted to see his past self-while the Fifth is convinced this man is just a ‘fan’ who managed to sneak into the TARDIS. However, the collision of the two TARDISes threatens to create an anomaly. The two-thanks to the tenth remembering what he did at this moment when he was the fifth-manage to cancel it out. The two share some talk about what was and what will be before the Fifth Doctor fades back into the past.
Peter Davison is actually David Tennant’s father-in-law in real life-Tennant is married to Georgia Moffet, Davison’s daughter.
However as the Doctor once again begins to take off, something unexpected occurs once again-the bow of a ship-the RMS Titanic-breaks into the TARDIS!