The Doctor takes his new companion Peri to Astroville Seven, a space station in the 31st century, where she can acclimatise herself to the wide variety of alien life forms in the galaxy, come to terms with the death of Kamelion, and go shopping. She and the Doctor enter a souvenir shop only to find three thugs preparing to kill the shop's proprietor, Hok, and as they try to intervene a man claiming to be Sir John Falstaff arrives and sets off the fire alarm. The thugs shoot Hok before they flee, but before he dies he recites a set of galactic co-ordinates. In the ensuing confusion, however, Falstaff departs as well, leaving the Doctor and Peri to explain themselves to the police. The suspicious inspector Myra Jaharnus confiscates the TARDIS key and orders them to remain on the station until her investigation is complete. The Doctor takes the opportunity to research the dying alien's words, and comes to believe that he had learned the location of the legendary treasure of Rovan Cartovall, who vanished without a trace 5000 years ago, taking the wealth of his empire with him. The case is eventually dropped due to lack of evidence, and the Doctor and Peri are permitted to leave, but the Doctor's curiosity has been piqued, and he decides to travel to the co-ordinates quoted by Hok to see what is actually there for himself.

The thug Qwaid reports to his boss, the criminal kingpin Alpha, that Hok was killed before he could sell his information to anyone else, but before departing on his expedition, Alpha orders Qwaid to plant a bomb on Falstaff's ship just to be safe. Soon after Falstaff leaves the station, his ship explodes... but the position of the wreckage makes it clear that Falstaff was on the same flight path as Alpha, suggesting that Hok was not killed in time after all. Alpha berates Qwaid for his failure, and, furious and frightened, Qwaid decides that he's had enough and sabotages the ship's systems, expelling Alpha into space. He then orders his former colleagues, now subordinates, to open Alpha's private strongbox and see what's inside, but when they hear a regular ticking sound coming from within, Qwaid panics and expels it from the ship as well. Satisfied that he has disposed of all of Alpha's traps, he and the others head off to collect Rovan's treasure -- unaware that the strongbox has activated its internal power systems and is following them. Meanwhile, another ship is on the same journey; the Marquis te Rosscarrino and his niece Arnella, accompanied by Professor Thorrin and his assistant Willis Brockwell, have also purchased the location of Rovan's treasure, and hope to see their greatest desires realized. But they too are being followed.

The Doctor and Peri pick up a distress call on their way and pause to rescue Falstaff, who managed to reach an escape pod before his ship exploded. But they are then arrested by Inspector Jaharnus, who had hidden aboard the TARDIS before it left Astroville, convinced that the Doctor and Peri were not as innocent as they appeared. She now has proof that they have been withholding evidence, and demands that the Doctor return to Astroville to answer for his crime -- but the Doctor finds that a mental barrier prevents him, or anyone else, from operating the TARDIS controls. The ship thus completes its pre-set journey to the planet Gelsandor, where Shalvis, leader of the Seers, greets all those who have come in search of Rovan's treasure -- including tabloid journalist Dexel Dynes, who has followed the Marquis to find out what he's up to and broadcast the story to the population of the galaxy. Dynes will be permitted to observe the progress of the treasure seekers, who find that they must undertake a quest to prove their worthiness; only then will they receive the reward they deserve. The Gelsandorans will observe their progress and study their reactions to come to a better understanding of the human mind. Thorrin and the Marquis are upset that their greatest hope seems to have been reduced to the level of a foolish game -- and only the Doctor wonders why the treasure is supposedly still complete, if the quest has been in place for centuries...

The treasure seekers split up into three parties, Jaharnus and Falstaff accompanying the Doctor and Peri. Their first task is to find their way through a series of forest paths, in which none of the signs or guides tell the truth. The Doctor and Thorrin use logic to find their way through, while Qwaid -- after a disastrous attempt to take a short-cut -- uses a compass and trail markers. Once through the forest, the parties must cross a vast plain of coloured tiles, some of which are rigged to open up beneath their victims' feet and drop them into what could be a deadly trap. The Doctor works out that the safe route repeats the colours of the rainbow, but even then his party has nearly dropped from dehydration before they realize that the blazing heat of the sun is killing them. The Doctor realizes the danger just in time, and deliberately opens up the rigged tiles until he locates one that opens into a pool of water, where he and his friends rest until the sun sets and they can continue on their way.

Qwaid and his party cross the plain by walking on the hinges between the tiles, but Qwaid is at least bright enough to realize that the tests will only get harder as they go on. He and the others therefore kidnap the Doctor and Peri during the night, intending to force the Doctor to help them through the remaining traps. The Doctor, Qwaid and Drorgan will continue on the quest, while Gribbs takes Peri back to the ship as a hostage to ensure the Doctor's good behaviour. The Gelsandorans do not intervene, and Peri realizes that they have no intention of doing so whatever happens; the questers will succeed or fail on their own merits. Dynes tries to interview Gribbs upon his return, and when he refuses to help Peri to escape Peri realizes that he is entirely amoral, interested only in getting a good story. Fortunately for her, Qwaid contacts Gribbs to ensure that he's returned safely, and when he realizes what Gribbs is doing he forbids him from speaking to Dynes and orders him to keep Peri in good health to ensure the Doctor's co-operation. Dynes returns to his ship, unaware that for some reason his mobile floating cameras are not detecting the Gelsandorans as they approach the TARDIS with a cloud of raw plasmic matter. They have predicted the arrival of a great evil against which even their formidable mental powers will mean nothing -- but there is a way to stop it from destroying all that they have worked for...

Without the Doctor or Peri by their side, Jaharnus and Falstaff join the Marquis' party, despite the Marquis' and Thorrin's clear reluctance to join forces with potential rivals. As dawn breaks, they continue on their quest, and are forced to fight their way through a valley populated by carnivorous monsters. The Doctor, however, deduces that the untouched fruits are probably poisonous, and by smearing themselves with the juice of the berries, he, Qwaid and Drorgan are able to pass through the valley without fear of attack. Just beyond the valley is a river where all of the raw materials to build a raft are in place, but when the Marquis' party tries to cross they are attacked by a river serpent and Arnella is nearly killed. Jaharnus saves her life, barely ahead of Brockwell, and the Marquis is deeply grateful -- if Arnella were to die then all of his hopes would come to nothing. Jaharnus realizes that Brockwell is in fact in love with Arnella but is too shy and too aware of the great social gap between them to say anything. Their next hurdle is a staircase set into a fog-shrouded cliff, which seems to stretch on to an infinite height -- in both directions, as they find that they are unable to retrace their steps to the bottom. Eventually, however, they work out that although the fog and the smooth movement makes it nearly impossible to see, this is a moving staircase and they have been walking in place. Brockwell and Jaharnus walk downwards while the others remain in place and are carried to the top of the stairway, where they jam the treads, allowing the others to walk back up and join them.

Beyond the cliffs lies a mire inhabited by former seekers who seem to have given up all hope of completing the quest. Exhausted by their efforts, the questers camp down for the night, but receive no benefit from the rest. By the time dawn breaks they are too listless and despondent to move... until Jaharnus slips and cracks her knee on the ground, and the sudden pain clears her head for just long enough for her to realize that they have fallen victim to a mental trap set by the Gelsandorans and snap the others out of their lethargy. The Doctor, Qwaid and Drorgan are woken from their stupor by Gribbs' frantic calls, and when Qwaid realizes how close he came to falling victim to the trap, he orders Gribbs to pilot the ship and pick them up; he's had enough of the Gelsandorans' tricks and intends to bypass them. Gribbs takes off, but finds himself unable to fly over the testing grounds -- and before he can land again, a failsafe programmed into the navigational system by Alpha shuts down the ship, causing it to crash in the forest. Gribbs and Peri eject safely before it does so, but when Gribbs tries to locate and recapture her, a creature like a biomechanical unicorn appears as if from nowhere and drives him off. The unicorn, whom Peri affectionately names Red, takes her back to the TARDIS to relax. Meanwhile, what appears to be a meteorite lands in the forest, and when Gribbs gets back to the ship he finds to his horror that Alpha has returned...

Beyond the valley of despair lies a forest of fear, but Thorrin is fed up with the Gelsandorans' mind games and he sets the forest on fire to clear a safe path. The flames burn out of control and surround the party, forcing them to scatter, and although a sudden downpour extinguishes the fire, the questers have become lost in the forest, and are forced to face their worst nightmares. Arnella is trapped by thornbushes which provide her with delicious fruit, but she is then attacked by parasites which feed upon her blood and cannot escape without being gouged by the thorns. The Marquis finds himself lost and alone, a little man of no significance whatsoever; Thorrin is put on trial for ignorantly murdering the living trees of the forest; and Falstaff finds himself faced with a shadow of himself, an empty shell of a man with no identity of his own. Qwaid forces the Doctor to remain on the outskirts of the forest until morning, when it will be safe to cross. When dawn breaks the Marquis' party is reunited; they have survived their experiences, but it seems to have made the Marquis and Thorrin even more determined to reach their goal.

Another relatively simple puzzle opens the way to Braal, a village in the Country of the Enlightened. Here, Arnella and Brockwell finally get to talking, and Arnella admits that her uncle expects to find proof that their family is directly descended from Rovan Cartovall and that Arnella is the rightful heir to what remains of his empire. She is now aware that she does not wish this responsibility, even though her late father and her uncle have expended the family fortune to discover the truth. Before Brockwell can reveal his feelings for her, however, they are placed under arrest by the villagers of Braal, as are the Doctor, Qwaid and Drorgon. The villagers have renounced the pursuit of wealth, believing that money is evil, and they find the questers guilty of seeking to release Rovan's treasure upon the galaxy and sentence them to death. Before sentence can be carried out, however, the gangsters' ship unexpectedly arrives, scattering the villagers -- and Qwaid is stunned to find that it has been piloted here by an android into which Alpha had regularly uploaded his own brain pattern as a safeguard in the event of his death. The android Alpha is unaffected by the Gelsandorans' influence, and it rescues Qwaid and Drorgon from the villagers and questions Qwaid to determine what he knows about the location of the treasure. Once Alpha is certain that Qwaid has told it everything, it kills him to prevent him from betraying it again.

In the confusion caused by the ship's landing, the others manage to escape, but the Doctor, Jaharnus and Falstaff are separated from the Marquis' party. Arnella and Brockwell are appalled when the Marquis and Thorrin insist upon continuing their quest, although this means abandoning the others to the villagers. Just as the Doctor and his allies are about to be recaptured, however, Peri and Red arrive and drive the villagers away. The questers continue onwards to a cliff face, where a rainbow cast by a sparkling waterfall leads them to a cave system full of traps -- which all prove to be harmless, false or otherwise unarmed. At the end of the line, however, the questers must pass beneath a huge, hanging stone slab, and they are given no way to determine whether it is also harmless or whether the other traps were meant to lull them into a false sense of security. Is this a test of their resolve or their ingenuity? Before anyone can stop him, Falstaff walks beneath the slab -- and survives unharmed. When the Doctor, Peri, Red and Jaharnus join him on the other side, he finally admits that his real name is Preston Loxley III; in life he was a bland, worthless man who had inherited his fortune but had no real identity of his own, and he therefore adopted the fictional character of Sir John Falstaff. But now he has proven his own character.

The parties are reunited in an antechamber with four coloured doors, where Shalvis is waiting for them -- as is Dynes, who has been invited to witness the denouement. The time has come for the participants to select which treasure they desire. The green door will lead them back to their ships; the blue door will grant their greatest desire, if their need is urgent and their commitment total; the yellow door will lead them to Rovan's material treasure, if this is all they wish for the rest of their lives; and the red door opens upon the ultimate treasure. To the Marquis' horror, Arnella decides that she wants nothing to do with her supposed birthright; instead, she will remain with Brockwell, having realized that she loves him as well. The Marquis thus passes through the blue door alone, as does Thorrin, who is convinced that Cartovall had discovered the secret of immortality, with which he will be able to continue his pursuit of knowledge forever.

Alpha sends Drorgan and Gribbs through the yellow door, and after passing through a series of bulkheads they find themselves in a room filled with priceless gems -- but the treasure has been preserved intact by filling the room with inert gas, and Drorgan and Gribbs suffocate before they realize what is happening. Having located the treasure, Alpha decides to kill the others and return to collect it -- but before he can do so, Red attacks and destroys him, although the unicorn suffers fatal wounds in the process. Satisfied that justice has been served, Jaharnus chooses to depart peacefully, as does Loxley, Brockwell and Arnella, who have gained all they deisre already. Dynes also chooses to depart, having enough of a story to take to the news media. Before they depart, Shalvis reveals the fate of the Marquis and Thorrin, who have been placed in suspended animation, in a virtual simulation where their desires have been fulfilled; they will remain in their dream worlds for the rest of their lives.

Much to the Doctor and Peri's surprise, as soon as the others have left through the green door the dying Red transforms into Kamelion. A portion of his personality remained intact after his death due to his interface with the TARDIS, and the Gelsandorans provided him with raw plasma with which to create a new form for himself and make amends for his actions on Sarn. Kamelion dissipates, grateful for his second chance. To Peri's surprise, the Doctor then exits through the red door -- and when she follows him she finds that, like the others, they have returned to their starting point. As the others depart, however, the Doctor and Peri are shown a holographic message from Rovan Cartovall which confirms the Doctor's suspicions; Rovan left his empire because he felt stifled and bored by the limits of his power, and he sought to start a new life with nothing. The ultimate treasure is the Universe itself, and all of its limitless possibilities. As the Doctor and Peri return to the TARDIS and depart, the other seekers go their separate ways... and Dynes discovers that all of his recordings are blank. The Gelsandorans only told him that he could witness the quest, not that he could broadcast it to the population of the galaxy as entertainment.

Source: Cameron Dixon