The Doctor returns to the courtroom after a recess, given to allow him to mourn Peri’s death, shown in the previous block of evidence. The Doctor begins his defence, showing events from his future on the galactic liner Hyperion III, a ship taking a supply of rare metals from Mogar to Earth in the year 2986AD. The Doctor states that many of the passengers and crew will not survive the journey to Earth, for "[someone determined to] protect a secret hidden on the space liner... will become a murderer."’’
On the ship, an elderly man named Kimber thinks he recognises a fellow passenger as an investigator called Hallett. However, the passenger denies this, claiming that he is a mineralogist called Grenville. A trio of scientist passengers - Professor Lasky and her colleagues Bruchner and Doland - are alarmed that Grenville might be an investigator.
Edwardes, the communications officer, detects a craft close to the ship - the TARDIS - but is unable to get a reply. Suddenly, an unseen figure attacks him, and injects him with a syringe. He then uses the communication equipment to send a message to the TARDIS. On board, the Doctor and Mel pick up a Mayday message, stating '...perative traitor be identified before landing Earth.' They materialise within the Hyperion III's cargo hold, next to the Hydroponics centre. The Doctor tells Mel he can sense evil, and says they should leave, but they are seized by guards. They are brought before Commodore Travers - whom the Doctor has met once before. Travers denies sending a mayday signal, but wants the Doctor and Mel to remain on board, and confines them to passenger quarters. Travers hopes (from his previous experience of the Doctor's behaviour) that he will be able to find out who sent the fake mayday call.
The Doctor is convinced that whoever sent the message knew him, and wanted him on board. Mel suggests that the Doctor ask for a passenger list, in the hope of spotting a familiar name, while she wanders around the large liner in hope of finding the mysterious contact. Security officer Rudge takes Mel to the ship's gymnasium, where he shows her how to use the headphones and tape recordings to help her exercise. While she is doing so, Doland arrives and informs Lasky that someone has broken into their Hydroponics centre. As they rush off to find Bruchner, Mel hears someone on her headphones, telling her to take the Doctor to Cabin 6. Down in the cargo hold, Lasky, Doland and Bruchner check the Hydroponics centre; the large pods inside are stable, but the Demeter seeds have been stolen from the small work cabin.
In the lounge, the Doctor persuades the stewardess Janet to let him see the passenger list, but he does not recognise any of the names. Mel arrives and gives the Doctor the message to go to Cabin 6, but the Doctor declines, claiming he feels like he would be simply walking into a trap. Mel decides to go by herself, but when she arrives, she discovers the Doctor is already there. They find the room has been wrecked, supposedly in a fight, and they find the silver Demeter seeds and a single boot.
Rudge contacts Travers to inform him there has been an 'accident' in the waste disposal unit; it seems that someone has been thrown inside. All that is left is a boot matching the one found by the Doctor and Mel in Cabin 6. They learn that these belonged to Grenville, but the Doctor does not recognise the name.
The Doctor and Mel go to the gymnasium. The Doctor says that, with Grenville dead, there is nothing they can do. Mel departs to investigate the hydroponics centre alone.
The Doctor claims that there have been changes to these scenes, and states that he is being manipulated.
Mel enters the cargo hold, where she meets Edwardes. He agrees to show her the Hydroponics centre. It was set up for the journey specially for Lasky, Doland and Bruchner, and that only 'low spectrum' light is allowed inside to keep the pods dormant. When Edwardes tries to enter, he is electrocuted, and sparks fly around the mysterious pods. Two guards arrive at the scene, and Mel tells them that Edwardes is dead. One of the guards leads Mel away, and the other guard examines Edwardes' corpse. Later, Doland and Bruchner arrive to find that all the pods have been opened. Bruchner looks on in horror, asking Doland if he realises what has been unleashed.
Rudge brings the Doctor to the bridge to help question Mel about being in the Hydroponics centre. Rudge then gets a message from the medical team that was sent down to the hold to collect Edwardes' body, claiming that neither Edwardes or the guard can be found. When Travers asks the Doctor what is going on - with a passenger, now Edwardes and a guard all gone missing, two, if not all three, murdered - Mel replies that a killer must be on board the ship.
Travers decides to speed up their journey to Earth, and has the ship's course altered. He enters the lounge to inform the passengers that their arrival time will be brought forward by around 72 hours. Three Mogarians express their concerns that this will take them close to the Black hole of Tartarus, but Travers assures them that they will be within adequate safety margins. Later, one of the Mogarians collapses after having a drink. The Doctor attempts to remove the figure's face plate, but the others protest that oxygen is lethal to a Mogarian. The Doctor believes it is not a Mogarian, and removes the face plate to reveal that is actually Grenville. The Doctor, however, recognises the man as Hallett, an undercover investigator. When Kimber recalls recognising Hallett before, in front of most of the other passengers, the Doctor guesses that Hallett faked his own death to avoid being discovered.
The Valeyard asks the doctor how he knew the man was a fake. The Doctor replays the scene where the Mogarians spoke to Travers about the black hole, which shows that the fake one did not have his translator switched on. The Doctor also points out that Hallett's death meant that he could no longer play a passive role on board the Hyperion III.
Mel realises that the Demeter seeds left in the wrecked cabin for her and the Doctor to discover were a clue to lead them to the Hydroponics centre. They go down to the hold to have a look at the place, and the Doctor wonders what came out of the pods. Returning to the passenger quarters, they see Lasky leaving a guarded Isolation Room. They wonder what she was doing inside, so the Doctor sets off a fire alarm to distract the guard. He and Mel enter the room, where they find a half-human, half-plant hybrid strapped to a table. The creature sits up and implores the Doctor and Mel to stop Lasky, but Lasky, Bruchner and Doland arrive and sedate her. Doland tells the time travellers that the creature is his assistant, Ruth Baxter. During their experiments involving cross-fertilisation, a speck of pollen penetrated a tiny scratch in Ruth's thumb, causing the resulting plant maturing process to partially transform her human body. They are taking her to Earth in the hope that they can reverse the infection.
Mel hears a noise in the air conditioning ducts. Attaching one of the headphones to the grill, she uses the gym sound equipment to amplify the sound and record what she hears onto one of the music tapes. She overhears creatures planning to kill all the 'animal-kind' on the ship. As she listens, she is attacked from behind and rendered unconscious with an anaesthetic. The murderer dumps her in a disposal trolley. The Doctor enters the gym and hears the recording, including her scream when she was attacked. The Doctor runs after the trolley, and rescues her from being killed in the waste disposal unit. Unfortunately, the killer has removed the tape.
Bruchner is becoming increasingly hysterical about the situation with the Hydroponics centre, especially when the elderly Kimber disappears. It turns out that he, Edwardes and the missing guard have all been killed by plant-like creatures called Vervoids - the creatures that came out of the pods when Edwardes was electrocuted. The Vervoids are slowly skulking through the Hyperion III's air ducts, secretly killing the crew and passengers one at a time, and dumping their bodies in their lair. Lasky finds Bruchner burning the notes on their work in the Hydroponics centre's small work cabin, and tries unsuccessfully to reason with him about his actions. Bruchner knocks Lasky out, runs off and attacks a guard, taking his gun. He goes to the bridge, and forces Travers and the pilot to leave, then changes the course of the Hyperion to head straight into the black hole of Tartarus - planning to destroy the ship, and the Vervoids with it.
The Doctor, Lasky and Travers attempt to break into the bridge, but they discover it is filled with marsh gas. This has been released into the bridge by the Vervoids, who - having overheard Lasky talking with Bruchner in the Hydroponics centre - have learned that they are the only members of their species. Bruchner is killed by the gas, but the ship is still heading into the black hole. Rudge summons the two Mogarians, as they can breathe in the poisonous atmosphere. They manage to direct the ship away from the black hole, but when it is safe, Rudge tells Travers that he and the Mogarians are taking over the ship.
Mel manages to get to the lounge ahead of Rudge, and warns Doland and Janet of the hijacking. Rudge tells the Doctor that the Mogarians are trying to regain the supply of metals stored in the vault, as they consider the expiring resources of rare minerals were stolen from their planet. Rudge himself is taking the hijacking as a means of securing a "more comfortable retirement", as this Mogar-Earth journey was to be his last voyage as a security officer before being written off. On the bridge, an unknown assailant kills the Mogarians by throwing liquid oxygen at them.
Mel sneaks through the air ducts to let the Doctor know that the guards will attack the lounge. The Doctor believes this is too risky, and tells her to attack the bridge instead. When they arrive, they find the Mogarians dead, and take the face plates to prove to Rudge the hijack is over. Doland knocks the gun from Rudge's hands, and he runs into the corridors, but is soon killed by the Vervoids.
The Doctor tells Travers about the stolen tape recording, and requests his permission to search the passenger cabins. While Mel checks Lasky's locker in the gym, the Doctor tells Doland that he thinks the traitor is either him or Lasky. After searching the professor's cabin, Doland suggests the cabinet in the Hydroponics centre work cabin. There, Doland reveals the tape is in his pocket, but that he has wiped it. Taking the Doctor's gun, Doland admits the murders. He reveals that he plans to use the Vervoids for slave labour, taking over factories and farms from robots. However, the Doctor has disarmed the gun, and Travers arrives and arrests Doland. However, he and his guard are attacked and killed by the Vervoids.
The Doctor, Mel, Travers and Lasky meet on the bridge to discuss the Vervoids. Lasky believes that something must have gone wrong with their DNA, but the Doctor tells them that the Vervoids' hostillity towards them is instinctive: They are a form of carefully bred plant life, and all 'animal-kind' ultimately consumes plant life. This means that the Vervoids hate 'animal-kind' and kill for survival. Lasky realises that this is what made Bruchner so hysterical, and vows to help destroy the creatures, with Travers asking the Doctor to do the same.
The Doctor states that this shows he was not meddling, but had been asked to help. The Inquisitor accepts his argument, but the Valeyard wishes to see the outcome of events.
In the hydroponics centre, Lasky finds that the necessary chemicals to create herbicide had been taken by the Vervoids. She, Mel and the Doctor are then surrounded by the plants. Lasky tries to reason with them, but they kill her and take her body back to their lair. Escaping through the air ducts, Mel and the Doctor discover the pile of bodies. The Doctor tells the distraught Mel that this is the Vervoids' version of a compost heap.
The Doctor has an idea that vionesium, the rare metal taken from Mogar stored in the ship's vault, would accelerate the Vervoids' life-cycle towards its natural end. Exposed to oxygenated air, vionesium releases intense light and carbon dioxide - to the Vervoids, the basic ingredients for their life-cycle induced by the seasons of spring, summer and autumn. Travers lowers the lighting and heating in the ship, forcing the Vervoids back to their lair, where the Doctor and Mel are waiting. They successfully deploy the metal against the Vervoids, which causes the creatures' leaf-covered bodies to turn brown, wither, and die. Having saved the survivors, the Doctor and Mel bid their farewells to Travers and Janet, and depart in the TARDIS.
The Inquisitor asks the Doctor if any of the Vervoids survived, and he informs her that none did; if even a leaf had survived and reached Earth, a Vervoid would have grown. Seizing on this, the Valeyard accuses the Doctor of committing genocide.
Doctor Colin Baker (Sixth Doctor)
Companion Bonnie Langford (Melanie Bush)
Lynda Bellingham — The Inquisitor
Michael Jayston — The Valeyard
Honor Blackman — Professor Sarah Lasky
Malcolm Tierney — Doland
David Allister — Bruchner
Michael Craig — Commodore Travers
Denys Hawthorne — Rudge
Yolande Palfrey — Janet
Arthur Hewlett — Kimber
Sam Howard — Atza
Tony Scoggo — Grenville/Enzu
Leon Davis — Ortezo
Simon Slater — Edwardes
Barbara Ward — Mutant/Ruth Baxter
Peppi Borza — First Vervoid
Bob Appleby — Second Vervoid
Mike Mungarvan — Duty Officer
Hugh Beverton — Guard/First Guard
Martin Weedon — Second Guard
Writer Pip and Jane Baker
Director Chris Clough
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 7C-1
Series Season 23
Length 4 episodes, 25 minutes each
Originally broadcast November 1–November 22, 1986