Contact with an oil rig somewhere off the shores of Scotland is interrupted by a strange sound and interference. Something impacts with the rig resulting in it crashing into the sea.

The Doctor, Sarah and Harry arrive in Scotland in response to a recall message from the Brigadier. On being told about the rigs being destroyed, the Doctor is not impressed, however the fact that lives have been lost sways him into offering his assistance. Remains of the leg from one of the rigs hint that it was bitten through.

Harry goes to the rescue of a survivor washed up on the beach, but is shot at. When Sarah stays at the hospital to check on Harry she is attacked by some kind of creature. It is not long before the Doctor uncovers a trail leading to a group of shape changing aliens stranded on Earth, and the legendary Loch Ness Monster...

Answering a distress signal, the Doctor and Sarah Jane Smith arrives on the planet of Zeta Minor. As they make their way through the jungle to the source, something passes by, something that makes Sarah feel as if she is being drained of her very existence. When they finally reach the hut from which the signal is emanating, the Doctor and Sarah Jane find the skeletal remains of a man next to a row of graves and an emitter with its battery nearly drained - all suggesting that whatever catastrophe struck took place a long time ago: they are too late to help anyone. Not sure where they are, the Doctor concludes that he needs an instrument from the TARDIS. Sarah offers to fetch it while the Doctor fixes the signalling device.

A probeship from Morestra goes into orbit around Zeta Minor: they are there to find out about Professor Sorenson and his team; the professor has been researching alternative sources of energy that would help fuel their world and empire for centuries to come; there have been no reports from the expedition for months - no-one knows if they are alive or dead. If there are hostile forces on the planet the crew of the probe have the capacity to eliminate them. The landing party, led by Vishinsky discover the TARDIS and have it tranported back to the probe for further investigation. As they move on through the forest they find Professor Sorenseon: his main concern is that he believes he has found what was looking for; a discovery vital to the survival of the Morestran Empire. Confronted with the fact that the rest of his expedition is dead - there can be only one explanation: this mysterious Doctor and Sarah are the killers...

Professor Marcus Scarman has found an untouched tomb that appears to date back to the first dynasty of the Pharoahs. Even the reliquaries are still sealed! Behind a tapestry Scarman finds the entrance to the inner chamber. On the door is the Eye of Horus: Scarman's Egyptian assistants leave the tomb in fear; he is alone and has to be open the entrance by himself.

Doctor Warlock is outraged that friends and family of Professor Scarman are not being allowed into his home. Namin declares himself to be caretaker while Professor Scarman is dealing with finds in Egypt. The only problem is that no-one has seen Scarman in months. A scream from a locked part of the house alerts both of them: Warlock is horrified by what he sees; Namin believes that the ancient gods have arisen and it is time to destroy all unbelievers. Fortunately for Warlock, the Doctor and Sarah are also in the house and save him from being killed. Some powerful energy force drew the TARDIS off course and has brought it to the old priory at the start of the 19th century. In years to come it will be the site for UNIT - their original destination. As the Doctor, Sarah and Warlock hide they uncover a scheme to restore the freedom of a powerful Egyptian god, in reality an alien from some distant planet. If this alien breaks free from his prison, nothing in the universe will be safe.

The village of Devesham: Sarah visited it a few years back to cover a story about a local space launch. The launch ended in tragedy: the ship and its pilot, Guy Crayford vanished. Now it seems that something else has happened: her return, with the Doctor, begins with helmeted men in white suits firing at them. Taking cover in the forest they then witness a soldier stagger over the edge of a quarry to his death. Examining the body they find no ID, but he is carrying freshly minted coins, no old currency at all. Eluding the white suited figures, the Doctor and Sarah make their way to the village itself. It is deserted, even the pub is empty. Again, the coins are freshly minted, even the dartboard is new. Recalling a reading of high background radiation the Doctor wonders if there has been some kind of accident.

A truck stops outside and a group of villagers sit quietly in the back, then they all get down and take their places, including sitting at tables in the pub or standing at the bar. As the clock strikes the people start talking and acting as if nothing strange was happening. The Doctor decides that he will go to the nearby space centre and contact UNIT from there. Sarah will remain to see what else happens in the village: among those who have entered the pub is the soldier she saw earlier plunge to his death...

The planet of Karn is not one that people choose to visit: ships crash there and those that survive the crash end up being butchered and parts of their bodies are used by Dr Mehendri Solon in his various experiments; the Sisterhood that live on the planet have little tolerance for visitors either - in fact it is their power that causes ships to crash; and they have no time for Time Lords. When the Doctor finds himself on Karn he surmises that he has been sent there: he no longer wishes to be a puppet for the Time Lords, carrying out their dirty work. A storm sends the Doctor and Sarah scurrying for shelter - to the nearby home of Solon. His pleasure at having company becomes overbearing, especially when he is fascinated by the Doctor's head. After consuming some of the food and drink provided by Solon, the Doctor loses consciousness. Sarah pretends that she too has consumed the evidently drugged food and drink.

The Sisterhood learn that a TARDIS is on Karn: although there was once an alliance between them and the Time Lords there has long been a degree of enmity. The Sisterhood depend upon the elixir for their longevity: now that elixir is in short supply and they suspect that the Time Lords have come to steal what remains. They must stop the Time Lords and not even taking refuge with Solon will protect him...

Scientists at an outpost in Antarctica excavate a seed pod in the permafrost. Given the depth it was found at it can only have been there for millions of years. As they try to thaw it out for further examination, the pod comes to life and a shoot attaches itself to a nearby scientist, Winlett. Information about the seed pod is leaked to millionaire Harrison Chase: a man who is fanatical about plants and the seed pod is the most unique find in history - and he must have it. UNIT is advised about the seed pod and also about the mysterious infection, so the Doctor and Sarah go to Antarctica. The Doctor guesses that there might be another seed pod nearby and sure enough he uncovers a second pod. Given what happened with the first pod, he advises that the second one be kept in cold storage. Meantime he tried to fnd a solution to the infection already changing the scientist. Concluding that the only possible answer is to amputate the infected arm before the mutation spreads further, the Doctor persuades the other scientists that is the only hope for both Winlett and the world.

Two men, Scorby and Keeler arrive, but they are not the requested medical team. They report that they lost their way in the bad weather and have landed there by chance. However, it becomes apparent that they have been sent there to recover the seed pod and they will go to any lengths to get it...and to conceal its disapperance. They also ignore the Doctor's warning that the seed pod could result in the extinction of all animal life on the planet.