Having missed the opening of Brighton Pavilion (again), the Doctor, Romana and K9 give consideration to other recreational venues. Among the more intriguing of the leisure planets (and one of the first on the list) is Argolis which, after a short war, built the Leisure Hive as a way of bringing people together. The Hive promises different environments enabling people to enjoy such pursuits as non gravity squash, but perhaps most interesting of all is their research into tachyonics.

Despite all it has to offer, the Leisure Hive is struggling financially. A generous offer from their former enemy, the Foamasi appears to be their only solution. Pangol, youngest of the Argolin, believes that their experiments with Tachyonics is the best way forward. For the Argolin, the Tachyon Recreation Generator might also be the last hope for their race as Hardin, an Earth scientist has promised that he can use the device to rejuvenate people. A visitor dies using the Tachyon Recreation Generator: other incidents point towards someone trying to sabotage the Hive. Without valid credentials, the newly arrived Doctor becomes the prime suspect.

General Grugger and his men have arrived on the barren planet of Zolfa-Thura. They have responded to a request to bring an Earthling to the Screens of Zolfa-Thura. At first, it appears as if there are only the Screens on the planet, but then a power complex arises from the sand within the area surrounded by the screens. Inside the complex they meet Meglos, the last of the Zolfa-Thurans. In order for his scheme to work Meglos first of all needs the Earthling - to use his body so that he can move away from the complex. His next step is to go to the nearby world of Tigella. For many years the people of Tigella have depended upon a mysterious energy source, the Dodecahedron - and Meglos needs that energy source for his own purposes.

The Dodecahedron is no longer stable, its power output fluctuating. For the Tigellans it could mean the difference between life and death. They live under the surface of the planet as the surface is quite hostile. The Tigellans are separated into two groups: the Savants, who follow science and logic and believe that they must study the Dodecahedron not only to understand it but also to better harness its power; and then there are the Deons who believe it is a gift from their god, Ti - and they will not countenance anything which is not an act of faith or worship. To the Deons, scientific study is blasphemy. However, they all depend upon the Dodecahedron and they need help. Zastor, who presides over both groups, decides that perhaps the help of an outsider is required. Years before, the Doctor visited Tigella and now he is in the vicinity. The Doctor can bring both science and faith together and he may be the sole means by which the Dodechahedron can be repaired. The Doctor's presence is also of interest to Meglos - the perfect means to stealing the Dodecahedron.

Assigned to help the Doctor in his quest for the Key to Time, Romana's extra travels with the Doctor have been a blessing. She has enjoyed her travels, but now it appears her journey is about to end: the Time Lords have summoned Romana to return to Gallifrey. The course is set, Gallifrey their next destination and while the Doctor and Romana have a heart to heart talk, K9 is left to monitor the TARDIS flight. En route to Gallifrey, something happens, something which K9 is unable to provide an explanation for. The TARDIS materialises, the scanner displays the Capitol of Gallifrey - it is time for Romana to bid farewell. Except, that when the time travellers emerge they find themselves near a forest. Nothing about their environs is reminiscent of Gallifrey. The scanner and the co-ordinates confirm their location as Gallifrey which suggests a possibility that neither the Doctor nor Romana care to acknowledge fully - the scanner reads positive co-ordinates. What if they have materialised on the negative equivalent of those co-ordinates? What if the strange, inexplicable phenomenon they experienced was a Charged Vacuum Embointment (CVE) - a doorway between our positive co-ordinate universe and the negative co-ordinate universe of E-Space? If this is the case, then they may be trapped in E-Space. They need local knowledge to help them make a more informed decision.

The people of the Starliner are harvesting river fruit; Dexeter is studying some samples of river fruit and notes the signs which he feels signify the coming of Mistfall. Once Mistfall comes, everyone will have to return to the Starliner and close the hatches until the atmosphere becomes breathable again. Not everyone intends to return though: some of the younger population want to live outside the system, they call themselves the Outlers. Adric wants to join the Outlers but they are unwilling to let him join: his brother Varsh is their leader but he cannot show favouritism; other members feel that Adric acts like one of the Deciders. He must complete a challenge in order to be considered as a new recruit: Adric must steal some riverfruit. Adric's attempt to steal the riverfruit coincides with the appearance of Decider Draith, leader of the Starliner community, and the beginning of Mistfall. Although Adric does steal a river fruit he is pursued by Draith. Most of the outlers return to their hideaway in a cave, others join with the rest of the community in returning to the Starliner. When Draith catches up with Adric, there is a scuffle next to the river and Draith is injured. Something drags Draith into the river and Adric escapes to find shelter. As he stumbles through the forest Adric finds the TARDIS. At first, believing Adric to be delusional, the Doctor and Romana soon conclude that they must investigate Mistfall and the Starliner community more fully.

The Three Who Rule send out Habris and his guards to make a selection: Aukon is hoping that this time they will find someone of spirit, someone who can join the Chosen Ones and serve the Great One at the time of Arising. Ivo, leader of the village, selects those who will be presented to the guards. He tells his son, Karl to stay in the shadows near the back of the hut. When Habris arrives though, he spots Karl and selects him. All must obey the Three, there are no exceptions.

K9 detects a habitable planet at extreme detection range. The planet is inhabited, high concentrations of metals suggest a high level of civilisation but low energy levels suggest that life is at a primitive level. Perhaps the society has been and gone? Arriving on the planet, the Doctor and Romana visit the village. Learning of the three lords who rule, they decide to investigate further. Ivo contacts Kalmar to inform him about these strangers and their questions about scientists. Kalmar's men intercept the Doctor and Romana and bring them to their hideaway. From Kalmar, the Doctor and Romana learn that science is forbidden and that it is the Three Who Rule who maintain this rather stagnated society. Even more interesting is the fact that the equipment in the hideaway is from Earth and they discover that the people are descendants of an Earth ship which crashed there many years ago. Perhaps the way back to their universe lies with either the computer records...or the Three Who Rule?

Rorvik and his crew are trapped between the time lines. Their navigator, Biroc, does not seem to be hleping, so a little encouragement is used to force him to choose a way out of the mysterious void. Biroc focuses on another object traversing the time lines: the TARDIS. With the ship still trapped, Biroc collapses. Rorvik orders Biroc to be taken below for some rest. The navigator takes advantage of this to make an escape, outside of the ship and into the void.

In their continuing attempts to escape from E-Space, the Doctor and Romana discuss various methods. One method, suggested by the Doctor, alarms Romana as it could potentially destroy the TARDIS. Adric tries another variation on the Doctor's idea and the TARDIS appears to stabilise. Then powerful forces batter the ship, the doors open and the time winds sweep through into the control room, damaging K9 before the Doctor can pull him out of the way. A figure emerges from the void, Biroc, and he stabilises the TARDIS before warning the travellers to beware of those who will follow. After Biroc exits the TARDIS, another question faces the travellers: the universe from which the Doctor, Romana and K9 originate has positive co-ordinates; the universe which they are currently in has negative co-ordinates. Now that Biroc has set them down at zero co-ordinates, could they be close to the way home? Intrigued, the Doctor sets off in pursuit of Biroc leaving Romana and Adric to face the new visitors: Rorvik, Packard and Lane - also in pursuit of Biroc, also determined to find their way out of the void. All they require is a navigator, preferably Biroc, but Rorvik determines that Romana may serve the purpose just as well.

Traken: an ideal world which enjoys peace and tranquility; all maintained by the Keeper in symbiosis with an energy known as The Source. However, even Keepers die and the old Keeper's time is nearing. His successor, Tremas, has been chosen and so it would seem that everything will be all right. Tremas was chosen as the Keeper-elect on his wedding day, his new wife, Kassia was the only person who was unhappy about this choice. Melkur, meaning a fly trapped in honey, was a malevolent force that came to Traken many years before. The power of the Keeper and the Source have held it in check and its evil has drained away over the years. When younger, Kassia was appointed to tend to the Melkur, to ease the pain of its passing. With her wedding to Tremas, the time has come that she must relinquish the task to Tremas' daughter, Nyssa.

Everything appears to be in hand, so the Doctor wonders why the Keeper has come to ask for his help. Now that the Keeper is dying, a time of transition is upon Traken. Forces that would normally have been kept at bay are beginning to manifest themselves. Evil has come to Traken and the Keeper wants the Doctor to help ease the transition from the old Keeper to the new. The problems begin when the Doctor and Adric arrive on Traken and are suspected as being the source of the evil. Even the Keeper seems powerless to help them now. The Doctor is troubled by the fact that there is something familiar about the evil he must now confront...

Apart from a short period in Earth history, the police box exterior of the TARDIS has become distinctive, something which concerns the Doctor. It is time that he repaired the ship's chameleon circuit, allowing it to once more blend in with its surroundings. To this end, the Doctor and Adric travel to Earth to find a real police box. The Doctor will map its dimensions and take that information to Logopolis: the people there have a unique mathematical ability that allows them to create or recreate matter from mathematical information. What begins as a simple task takes on a more sinister edge when a gravity bubble affects the TARDIS. One possibility is that another TARDIS has materialised around the police box, just as they had intended. Another TARDIS suggests an old enemy of the Doctor's has anticipated his plan. There can be no question of going to Logopolis.

A glimpse into the future persuades the Doctor to continue on to Logopolis. It is not long before the Doctor discovers that he does indeed have extra passengers on board the TARDIS. Tegan Jovanka, an air stewardess on her way to her first day on the job, the victim of a punctured tyre, had entered the TARDIS believing it to be a real police box. Lost within its interior she has only now found her way back to the control room. Tegan must face the fact that her kindly aunt, Vanessa has fallen foul of the ship's other trespasser - the Master. The Doctor soon discovers that the Master is not just after revenge, he has come to Logopolis in order to find out what great secret the Logopolitans are hiding. One that might just given him control over the universe...