The Doctor and his companions Ian, Barbara and Susan arrive on what appears to be an uninhabited island on the planet of Marinus. The island is surrounded by an acidic ocean, and features a large tower.
After the arrival of the Tardis, several torpedo-like ships land upon the island, which are carrying as passengers several members of the Voord. The Voord are interested is taking control of the Conscience of Marinus, a mega-computer that for generations was the final word of law and order on Marinus.
Arbitan, who guards the Conscience, had split up the five keys to the Conscience all over Marinus, to prevent the Voord from gaining control of the Conscience. Now Arbitan need to gather the keys to the Conscience to fight off the Voord. He's kept one of the keys himself and has sent out several assistants previously to gather the other four keys, including his own daughter, but none have returned. Arbitan, desperate, forces The Doctor and his companions to undertake the quest to gather the keys by keeping the travelers from reentering the Tardis via a forcefield.
We see several different environments during our travels over Marinus. The first is a city where every wish seems to be fulfilled. In reality, it's all an illusion created by several aliens who have the citizens under their control by hynopsis. Barbara is the first to break the hynopsis link, and eventually they are able to beat the aliens and retrieve the first key. They also discover that Arbitan's daughter, Sabetha and one of Arbitan's assistants, Altos as well. Altos and Sabetha join The Doctor and his companions on their quest to retrieve the rest of the keys.
At the end of this first quest, the decision is made to split up the group. The Doctor travels to the location of the last key, while the other travelers make for the second location. The second location takes place in a jungle environment and the third takes place in an arctic environment. After the retrieval of those two keys, the other travelers meet up with The Doctor at the last location, the city of Millenius. A complication ensues when Ian is framed for murder and the need for The Doctor to prove Ian's innocence.
Once all four keys have been gathered, and the real murderer has been revealed and Ian's innocence proved, the travelers return to the original island and the tower. The Voord have killed off Arbitan and are now in control of the tower. Ian gives the Voord a fake key that they found during the jungle adventure. The Voord uses this fake key and it ends up destroying the Conscience, the Tower and killing the Voord as well.
Altos and Sabetha have decided to get married, and leave The Doctor and his companions. The Doctor, Ian, Barbara and Susan, the forcefield around the Tardis now removed, resume their travels.
The format of "smaller quests inside a larger quest" was an intriguing idea. It certainly kept my interest more than the previous serial, Marco Polo, did. This story certainly was an inspiration for the much later "Key to Time" season-long arc from the Fourth Doctor era. Some of the mini-quests were more enjoyable than others, particularly the last adventure where The Doctor has to defend Ian against murder charges. A courtroom drama isn't the first thing one thinks of when thinking of Doctor Who, but it's interesting to watch and it proves that Doctor Who can do pretty much anything, and it's still Doctor Who. This serial is definitely a winner in my book.
