Donna Noble is looking into Adipose: the latest company selling a pill which it claims is the quickest way to lose fat. After a visit to the call centre area, Donna manages to obtain a client list. Paying a visit to one of the clients soon reveals that there is something far more sinister about the quick fix pill than meets the eye. When she returns home, Donna is nagged by her mother and so she leaves again to talk to her grandfather. One day, a hundred years from now, he believes humans will be living on other worlds like Venus. Donna asks him to keep an eye on the skies and to give her a shout if he sees a little blue box flying across the sky.

The Doctor is investigating Adipose, posing as a health and safety inspector. After a visit to the call centre area, the Doctor obtains a client list and visits one of the clients. While the client is amazed at how effective the treatment is, he is annoyed by the fact that he is woken up at 1:05 am every morning by his intruder alarm. Perhaps its a cat as something seems to make use of the cat flap.

Both Donna and the Doctor return to Adipose, determined to find out what is going on, each unaware that the other is there, each about to find out the exact nature of Adipose and what it means to the obese people of Earth...

Rome: at its height one of the greatest cities of the world and the heart of one of Earth's great empires. Except, as Donna points out, instead of the seven hills there is one big mountain. The local population are not too concerned about an earth tremor either. The Doctor is concerned though as he realises that they have not arrived in Rome but in the city of Pompeii and the mountain is in fact Mount Vesuvius...and history says it will erupt and wipe out the people of Pompeii.

Donna suggests that they round up the people and tell them what is about to happen, but the Doctor states that no one would believe them. In fact, they would probably be regarded as crackpot soothsayers. One soothsayer overhears the Doctor and communicates with her sisterhood. It would appear that there is a prophecy and according to that prophecy Pompeii will endure forever...

Selecting a world at random, the Doctor brings Donna to her first other planet. Her enthusiasm is dented by the icy weather and then it is tested further by the discovery of a dying Ood. Despite his efforts the Doctor is unable to help the Ood - but it makes a remark about "the circle" and displays the same red eyes as other Ood did in a previous encounter. The travellers move further along and find their way to a nearby facility. A tour is taking place as the facility is entertaining new customers with a view to selling on new batches of Ood. Donna is horrified to discover that the Ood are nothing more than a slave race.

Mr Halpen, the Chief Executive of Ood Industries, is concerned about the increase in this red eye phenomenon: some of the Ood are becoming quite rabid. His efforts to contain the infection are not working as well as they could do. Could there be some connection between the infection and the contents of Warehouse 15...?

The Doctor and Donna soon begin to unravel the nature of the red eye sickness and the horror that has befallen the Ood...

Jo Nakashima is thrown out of the Rattigan Academy. As she drives to UNIT via her ATMOS satellite navigation she calls in her concerns about ATMOS. When the sat nav advises her to take a right turn after the bridge Jo is worried to find her car facing the river. The gears automatically engage and the car plunges into the river.

The Doctor is giving Donna some lessons in operating the TARDIS when a telephone call comes through from Martha Jones. Returning to Earth, the Doctor watches as UNIT seizes a nearby ATMOS factory. There would appear to one problem though - nearly every car on the planet carries the ATMOS device and it has a zero rate of carbon emission. So it must be of some benefit to the planet? The Doctor has been brought to Earth to investigate the nature of the ATMOS device - since 52 people around the world, died at exactly the same time - and their cars all had ATMOS devices.

UNIT soldiers are sealing off the factory and rounding up workers. Two soldiers find two workers guarding a restricted area. Once inside the restricted area, the soldiers discover that ATMOS is hiding something else...

Some force has dragged the TARDIS through space and time to the planet of Messaline. The Doctor, Donna and Martha emerge to find themselves in a tunnel and they are met by a small band of armed humans. One thrusts the Doctor's hand into a progenitor machine which extracts some of his DNA and uses it to create an instant soldier. The female soldier that emerges is programmed to fight and when the enemy, the Hath arrive there is a quick skirmish. Martha is captured by a Hath but before the Doctor can do anything to help, the humans detonate part of the tunnel sealing themselves off from the Hath controlled section.

Taken to meet Cobb, the leader of the human faction, the Doctor and Donna discover that the humans and the Hath have been fighting a long war to gain control of the planet. Only one faction can survive and they intend to take control of the Source, the captured breath of the Creator. Everyone is a fighter and they have no time for pacifists like the Doctor. The Doctor must also confront the fact that the female soldier, named Jenny by Donna, is technically his daughter. How can he convince her that fighting and killing is not the way?

The Doctor and Donna arrive just before Lady Eddison's garden party begins. Posing as guests, the time travellers are thrilled to discover that another of the guests is none other than the author, Agatha Christie. The intrigue begins when the Doctor realises that they have arrived on the eve of Agatha Christie's 10 day disapperance. There is cause for further intrigue when a visitor is found dead in the library having been hit over the head by a piece of lead pipe.

As the guests begin to die one by one the Doctor must try and find the connection between a giant wasp and an unfolding mystery that could have come straight out of an Agatha Christie book. Who better to help solve the mystery than Agatha Christie herself?

When she closes her eyes, The Girl finds herself in a vast Library. Each time she sees herself in the Library she is in a different place, so vast is the Library. Then one day she closes her eyes, finds herself in the Library and something begins to break in. The doors crash open and in runs the Doctor and Donna. The Doctor and Donna have come to the Library apparently on a whim of the Doctor's. There is absolute silence, a silence greater than should be normal. Yet when the Doctor puts the appropriate question to the data core there are millions of lifeforms - all saved. Where are they?

A node passes on a message - a message warning them to run. Another node tells them to count the shadows. As the Library begins to plunge into darkness, the Doctor and Donna run. Their run ends in a room which leads to the shop. It is not long before they are joined - archaeologist Professor River Song has led an expedition to the Library which includes a descendant of the family which owns the Library. As the people prepare to defend themselves against whatever is in the Library with them, the Doctor discovers that there is more to Professor Song than she is willing to tell him...

Donna is enjoying a spot of sunbathing by the pool...under a dome of 15 foot thick glass which protects her from the lethal extonic radiation that bathes the surface of the planet Midnight. Despite all his attempts the Doctor cannot convince her to join him on a Crusaders bus tour to the Sapphire Waterfall. Leaving Donna to her book and sunbathing, the Doctor boards the tour bus and joins the small group of passengers as they embark on a trip that should allow them to see one of the most fabulous sights in the universe.

Reluctant to watch the in-flight entertainment or listen to the music of choice, the Doctor sabotages the entertainment equipment, forcing the passengers to talk to one another, to share their dreams and wishes for this excursion. Then a power failure forces the tour bus down to the surface. The driver and the mechanic try their best to restart the ship but ultimately they have to resort to activating a distress beacon. The Doctor joins them in the cockpit for a few minutes and convinces them to open the front shield so that they can gaze at the surface of Midnight. As they enjoy the fabulous vista, the mechanic swears that he can see a shadow like something moving...something that could be running towards the bus.

Returning to the passenger section, the Doctor reassures the other passengers that they will be rescued in time. Then something begins to rap against the side of the bus, moving along as if searching for a way in...

On a visit to a bazaar on a distant planet, Donna is lured into the shop of a palm reader. As the palm reader tries to discern more about Donna, about the decisions she has made in her life, about one particular decision that may have changed her life entirely, something scuttles across the floor towards Donna's back. In her mind, Donna recalls the day when her mother and her were driving into town. At the junction she turned left continuing with her job as a temp despite her mother's argument about going to see Mr Chowdry and perhaps accepting a permanent job. What if Donna had made that choice, what if she had turned right? As Donna reaches the crucial point something leaps onto her back...and she turns right.

On Christmas day, Donna is enjoying the fruits of her recent promotion when suddenly everyone spills out onto the street. Joining them, she watches as a giant star files overhead. Beams of light shoot out from the star and shoot destruction on the city below. Tanks fire at the star, destroying it - so all seems well. Donna runs towards the scene of the battle and watches as UNIT carry away a body, she overhears a soldier talking on his radio, advising that the Doctor is dead. This means nothing to Donna, but it means something to the blond-haired woman who walks up to her. As Donna's life continues, other problems begin to arise, new threats appear which seem to push the human race towards the brink of destruction. Throughout these moments, this strange blond-haired woman appears...and why do some people keep trying to look over her shoulder as if there was something on her back?

Arriving on Earth, the Doctor and Donna are relieved to find that everything is all right. So what is the significance of the warning they received? Returning to the TARDIS, the Doctor prepares to check everything. When the ground shudders Donna asks him what he has done. The Doctor checks outside to find that the TARDIS is hanging in space. A check of the instruments reveals that the Earth is gone - the TARDIS has remained in place while the whole planet has been stolen. There is only one course of action he can take - and so the Doctor sets course for the Shadow Proclamation.

Martha Jones at UNIT in New York, Captain Jack at Torchwood in Cardiff and Sarah Jane Smith at her home all wonder at what has happened. The sky has changed and there are many planets visible in close proximity. Something has moved the Earth and these other worlds into some other dimension. Martha attempts to call the Doctor but her signal is not getting through. Something emerges from the nebulous regions surrounding the planets, something that is heading towards Earth - a vast wave of ships. This vast wave of unknown ships is broadcasting a single message and as the world listens in, Martha, Jack and Sarah are horrified to recognise the cry of "Exterminate!". One other person is listening in and she too is wondering if the Doctor will arrive in time...Rose Tyler has crossed the gulf between universes to assist in the battle against the Doctor's greatest foe - the Daleks.

Arriving on Earth in 1851 the Doctor is confronted by an ideal Christmas. Until the shout of "Doctor!" causes him to break into a run. A woman by the name of Rosita has called him but is surprised to see him and explains that she is waiting for the Doctor. Before the Doctor can comment any further another man arrives on the scene, identifies himself as the Doctor - the one and only, and then asks for his sonic screwdriver.

The Doctor not only has to confront a Doctor he does not know - presumably a future self, but he must also deal with a nineteenth century London about to be taken over by an army of Cybermen. Behind this army is the mysterious Miss Hartigan and her hopes of assisting in the rising of the Cyber King.