When the Doctor and Gabby arrive in London, they are lured into a trap by an exiled alien who intends to use the Doctor's pain to open the door to its freedom.
The Tenth Doctor has briefly left Cindy Wu at Sunset Park to allow her to rest. The Doctor and Gabby are travelling in the TARDIS when they hit dimensional turbulence which activates the randomiser. After the TARDIS lands, the Doctor and Gabby exit, expecting a surprise, when he realises they are only in London. While Gabby is excited, the Doctor is less pleased, saying they need to leave. Gabby protests, and the Doctor gets angry, but is interrupted by a scream in the distance. Running to find the source of the noise, they arrive by the door of a stockroom, where a woman is talking to security guards, explaining that she saw a "monster" inside. The Doctor reluctantly agrees to stay for a short while, walking towards the stockroom. Elsewhere, a man watches them, talking to an unknown voice.
Walking through the stockroom, the Doctor asks Gabby if she knows when a stockroom isn't a stockroom. She doesn't believe he's being serious, so he reveals a doorway to another dimension, which shows a creature. He is being watched by a group of similar creatures. The creatures say that the Doctor will need their help against "the exiled". The Doctor tells Gabby that the creature isn't actually there, and the doorway is just a 4-D echo. The Doctor tries to lock onto the source of the dimensional bleed, and isn't able to find out what it was, but knows where to look.
The two of them arrive at The Key, a think-tank established by Aaron Crossland, which had been abandoned for six months. He unlocks the door, and they walk inside, as Gabby asks him why being in London is such a problem for him. He says that bad stuff always happens to him in London. They are interrupted by Crossland himself, who welcomes them to The Key. As he speaks, he begins to glow, and a door behind him opens to reveal a creature similar to the one they saw earlier in the echo, but larger and redder. The Doctor uses his sonic screwdriver, and detects the creature is giving off massive psionic power readings. He asks it what it is, and it calls itself the Reach. He asks it what it wants, and it says it wants to reach its home again. The Doctor asks it how, and Crossland says it will break his heart. He attacks Gabby, lifting her up in a glowing disc. The Doctor tells him to put Gabby down, but Crossland throws her out a door into the other dimension. The door slams as the Doctor tries to get through it. He asks Crossland where she is, and Crossland tells him she is in "the space between spaces", and that the Doctor must follow her in to be able to save her. The Doctor knows it's a trap, but opens the door with his sonic screwdriver regardless, and leaps in to follow her.
He lands on the ground, and looks around him. He sees who he think is Gabby lying nearby, and turns her over to find it's Martha Jones, who has been knocked out by a stun mine. A robotic figure walks up behind him, and attempts to incinerate him with blasts of fire from its hands, but the Doctor uses his screwdriver to push the fire back at it, killing it. As another "door" opens, the Doctor apologizes to Martha and leaves in it.
He exits the door in a house, hearing voices. Looking at photographs, he realises he is in the house of Donna Noble. Knowing he can't stay, he leaves through another door, and arrives at the Powell Estate, before it has finished being developed. As he collapses to his knees, Crossland and the Reach watch him, saying that the scenarios have activated the correct emotional response. The Doctor hears a voice behind him, and Gabby walks through a door. As the Doctor runs through, he asks her how she survived. She says some "familiar faces" caught her as she fell.
As Crossland and the Reach prepare to finish their plan, the Doctor and Gabby arrive through a door, revealing that the rest of the Reach's species helped them. The Doctor explains that he knows the Reach's plan, and asks the Reach whether it has anything to say. It grabs them with its tentacles and pulls them towards the door. As they get pulled closer, the Doctor tries to speak to Crossland, saying that the Reach's plan will not only destroy his species, but backfire and destroy half of London. Crossland slowly loses connection to the Reach as it protests. Crossland, now in control, apologizes and throws himself at the Reach. It lets go of the Doctor and Gabby as the door slams. As the two of them run away, exiting the building, the room behind them explodes, destroying the building but also sealing the dimensional link.
The Doctor and Gabby return to the TARDIS, and Gabby asks him what he would have done if he had lost her. Avoiding the question, he asks her where she wants to go next. They enter the TARDIS and it dematerialises.
- The Doctor tells the Reach it should have been called an "octonaut", but that the name was probably taken.
- While travelling through "the space between spaces", the Doctor finds an unconscious Martha Jones, visits Donna Noble's home, and, finally, finds himself in the (pre-developed) Powell Estate.
- The Doctor asks Gabby if she wants to see a light opera on Olphus 9, or visit the Wide Wing of the High Shansheeth Nest.
to be added
- The Doctor and Gabby have temporarily left Cindy on Earth after taking her home to give her "a little time to breathe" after Roscoe's death. (COMIC: Music Man)
- Gabby mentions meeting other incarnations of the Doctor. (COMIC: Four Doctors)
- The TARDIS randomiser is activated. (TV: The Armageddon Factor)
to be added