As the TARDIS approaches a planet Barbara hurts herself on the TARDIS console. Checking the control panel the Doctor notices that a component seems to be overheating. He asks Susan to check the fault locater. Just as Susan reads out the fault, the TARDIS doors begin to open...even though the ship is still in flight! An alarm sounds and the Doctor tells Ian and Barbara to close the doors.

Once the doors are closed and the TARDIS lands, the Doctor tells Susan to check the fault locater again. Ian and Barbara try to get an explanation from the Doctor. Susan explains that the moment of materialisation is the most dangerous - for the doors to open at that point...anything could happen. Yet the fault locater now appears to be showing no readings. Only the scanner appears to be at fault - exploding as it tries to scan the outside. The four travellers exit the TARDIS and find that they have arrived in some kind of canyon: the surrounding rock formations seem strange, as if comprised of different materials, one of which appears to be a rough type of concrete. Its not long before the travellers encounter other life within the canyon...or rather dead animals - giant worms, giant ants...they appear to have arrived on a world of giants.

A man with apparatus on his head stumbles through the weeds to the riverside. He tears off the collar attachment and then walks down into the river and out into the water. As his body is swept away, the TARDIS materialises on the riverbank, just under a bridge. Readings appear to show an Earth normal environment so the travellers leave the TARDIS to have a look around. Barbara and Ian are pleased to discover that they are in London, perhaps near the docks. The Doctor ponders over what era of time they have arrived and Susan climbs a nearby wall to try and have a better look around.

The general neglect and decay under the bridge is a cause for concern. Susan remarks that she cannot see anyone and then puts her foot wrong. Falling down she also causes part of the bridge to collapse, blocking access to the TARDIS. Ian and the Doctor set off to a nearby warehouse to find equipment: for one thing, Ian feels that they should at least have access to the TARDIS before they explore any further. The Doctor is also concerned that as time has passed they have heard nothing: no traffic, no bird, no people. Susan, unable to walk on her injured ankle, remains by the TARDIS. Barbara stays with her - she goes to wet a handkerchief in the river and spots a sign forbidding the dumping of bodies in the river. All the evidence suggests that it is not Earth of the 1960s.

The discovery of a body in the warehouse and the apparatus on his head, the sounds of guns, people on the run, hiding here and there and trying to avoid patrols, and finally a ship coming in to land - all point to Earth having been invaded.

Vicki is excited: the detector appears to show that the rescue ship has arrived. She informs Bennett - but he insists that she is mistaken, the rescue ship is not due for days yet. Instead, he suggest that Vicki investigates the source of the signal. Bennett also warns Vicki to be wary of Koquillion and not to let him know about the rescue ship. Vicki uses the communicator to contact the rescue ship - they confirm they are still 69 hours from Dido. So, what has landed on the mountain?

The TARDIS has arrived - and the Doctor has slept through the landing - much to the surprise of Ian and Barbara. They appear to have arrived somewhere nice and quiet, so perhaps they should go outside and have a look around. The TARDIS has materialised in a cave - the Doctor returns to the TARDIS to have a nap: Ian and Barbara set off to explore a part of the cave where the sun appears to be coming through. Outside, they look down upon the broken remains of a spaceship, close to some destroyed buildings. A being greets the travellers, or rather asks them some questions. While Ian goes to fetch the Doctor, the being approaches Barbara. Feeling threatened, she backs away and falls. Using a device he is carrying, the being causes the entrance to the cave to collapse, trapping Ian and the Doctor inside.

A month after the TARDIS materialised in a precarious position, the Doctor, Ian, Barbara and Vicki are enjoying a nice rest. Barbara and Vicki are going to the market at the nearby village. A man in the bushes listens to them as they walk past - but is more interested in maintaining the sharpness of his sword. Sevcheria and Didius are searching for further merchandise before they reach Rome. However, the village does not look promising...until they spot Barbara and Vicki. According to a stallholder, there are four of these new arrivals - they have been staying at a nearby villa (the owner is away on a campaign).

After dinner, the Doctor becomes irritable and states that he intends to spend a few days away. When he reveals that he is intending to go to Rome, Vicki asks to go with him. When Ian and Barbara ask to come along too, the Doctor objects and suggests that if they wish to go to Rome they should do so - by themselves.

Sevcheria and Didius arrive at the villa and after a struggle they capture Ian and Barbara: they will be sold as slaves. En route to Rome, the Doctor and Vicki find the body of a lyre player - evidently he was murdered. The strange thing is, if it was a robbery why has his lyre been left with his body? A soldier arrives - seeing the Doctor with the lyre, the soldier reveals himself to be an escort searching for the lyre player, Maximus Pettulian. On hearing that Maximus was due to perform before Nero, the Doctor decides to go to Rome.

During the split second of time during which the TARDIS materialised some force has trapped it and is now dragging the ship down to some strange planet. The air of the planet is thin and static. Vistas of crags, rocks and pools - all grim. What is the source of the power that has captured the TARDIS - is it natural or deliberate? Vicki is disturbed by some sound, which the Doctor determines is extra-sonic and that given her relative youth, Vicki alone can hear. Strange ant-like creatures surround the TARDIS and as it tries to dematerialise they direct a grub-like creature to surround the ship with some kind of field.

No power is getting through to the console. The Doctor and Ian decide to explore in the hope of finding the source of the influence. Equipped with atmospheric density jackets, the Doctor and Ian leave the TARDIS. Barbara remains to tend to Vicki who is still suffering the ill-effects caused by the extra-sonic noise. During their conversation, Barbara and Vicki discuss the bracelet worn by the former - a present from Nero. As time passes, something begins to exert an influence on Barbara via the bracelet, drawing her out of the TARDIS. After Ian becomes trapped, the Doctor rushes back to the ship for help...only to find the TARDIS gone.

A group of men stalk another group: they are waiting to find out if one the men making up the second group is the king. The king is indeed among the second group, out hunting with a hawk. Des Preaux insists that he can hear sounds in the woods and that they should not be here. No Saracens have been seen so the king insists that Des Preaux is being over cautious and fearing his own shadow.

The Doctor, Ian, Barbara and Vicki are in another part of the forest when a Saracen attacks them. During the skirmish, Barbara is kidnapped by other Saracens. The sound of nearby sword fighting forces the three remaining travellers to hide. They emerge to help an injured man - the Doctor remarks that the man who surrendered to the Saracens identified himself as Malik Rik - another name for King Richard, the lionheart. The injured man informs them that the man was not the true king. The Doctor reasons that if they help the man it might win them favour with the king and he will in turn help them to find Barbara. In the meantime the Doctor will try and obtain some clothes for them to wear so that they will not look out of place.

William des Preaux and Barbara are kept captive together: he reveals that he has posed as King Richard and that so far the Saracens have accepted this; Barbaa must therefore assume the guise of Joanna, the King's sister. For the time being they will be treated reasonably well, but as to what will happen when the truth is discovered...

The TARDIS crew stand around the console, frozen in time. The TARDIS arrives, the travellers return to normal...except that they are wearing their own clothes and not those they were wearing when they had boarded the ship. The Doctor dismisses it as some trick of time. Vicki checks the wardrobes - their medieval clothing is there. Vicki goes to the food machine for a glass of water but then is startled. Vicki drops the glass and it shatters on the floor, the water spilling out. Then time for the glass reverses and the glass unshatters and restores itself to Vicki's hand - the water no longer spilled, but in the glass. The Doctor is not concerned, but is intrigued by a nearby building - which looks like a museum.

Outside, the travellers walk across the dusty surface towards the museum - but they leave no footprints. Once they arrive at the museum, Barbara remarks on the silence. When the door opens, the travellers hide and despite herself, Vicki cannot help but sneeze. The two men who walk out of the museum continue on as if they had heard nothing. Taking advantage of the fact that they have not been noticed, the travellers begin an exploration of the museum. More people appear, conducting a conversation, which the travellers witness from their hiding place. The puzzling thing is that they cannot hear what the new arrivals are saying!

Despite the Doctor's earlier warning, Vicki can no longer resists the temptation to touch one of the artefacts in the museum...only for her hand to pass through it. As the travellers discuss this more men appear, again conducting a conversation which they cannot overhear. Even more disturbing, one man passes close to them and even looks at them...or rather through them. As the travellers continue to investigate, they finally ecnounter a very disturbing exhibit in the museum - themselves!

The Daleks are monitoring the travels of the TARDIS: they have detected it leaving the planet Xeros; the Daleks' own time machine will now be dispatched to intercept and destroy their enemies! In the meantime the Doctor makes use of one of the exhibits from the space museum, a space-time visualiser to entertain his fellow travellers. When the TARDIS materialises the travellers exmaine the local landscape. Vicki, excited hurries off to explore. Ian sets off after her - the Doctor gives him a magnetic device that will allow them to find the TARDIS. Barbara and the Doctor remain at the TARDIS to enjoy the warmth of the twin suns. A noise disturbs Barbara - she realises it is the visualiser and so returns to the TARDIS to switch it off. Howevere, in trying to turn it off she intercepts a message from the Daleks!

Realising that what they are seeing are past events, the Doctor and Barbara set off to find the other two - the Dalek extermination squad is either on its way or has already arrived! The Daleks have their instructions - to pursue the travellers through eternity and exterminate them. The chase through time and space begins!

The Doctor and Vicki are discussing recent events, when a noise in another part of the TARDIS alarms them. They prepare to tackle the intruder...who turns out to the space pilot they encountered previously - Steven Taylor. He collapses on the floor of the TARDIS. The TARDIS itself materialises on a beach. From the clifftop its arrival is witnessed by a monk.

As Steven recovers, Vicki explains about the TARDIS - but the pilot finds it hard to believe. As all the readings are normal, they decide to explore - Steven can then decide for himself if there is any truth in what Vicki says. A rusting Viking helmet does little to persuade him - after all, it could be part of a costume. It would seem that further exploration is required. The Doctor takes a path up to the cliff. Impatiently, Steven challenges Vicki to climb with him so they can meet the Doctor at the top. Once the travellers have gone, the Monk appears from his hiding place and smiles at the TARDIS.

The Doctor comes to a village and meets a woman: the men have left to look into rumours of an arrival. She provides him with a drink and the conversation provides the Doctor with the knowledge that it is the north of England in the latter half of the 11th century. The sound of monks singing from the nearby monastery does not sound right so the Doctor asks about the monks. Only one has been seen around the village. The Doctor decides to investigate - and what he finds may have repercussions for history.