This story is the first time since Susan in the first episode where we have a companion The Doctor has already been travelling with. It also has not happened since. The Doctor is still sad from the news of Peri's death. The Inquisitor has given him time for his distress and The Valeyard insists they continue. The Doctor is to present his defense. He has been allowed access to the matrix and plans to present evidence from his future, his defense being that he improves. Evidence where he thwarts a threat to a cruise liner and the entire planet Earth. The Hyperion 3, bound from Mogar to Earth, with passengers and a priority consignment of precious metals. It is 2986 and the last passengers board. One will murder. A blonde woman, Professor Lasky, complains her key doesn't work. Final passengers check in with Janet, the lone cruise stewardess. An old man calls out to the last passenger, recognizing him as an investigator named Hallett. The man denies it and claims to be named Grenville. Laskey and her two colleagues, Bruchner and Doland, are worried at the thought of an investigator. The Hyperion leaves Mogar.
In the TARDIS, Mel is making The Doctor work out on a stationary bike, then offers him carrot juice to quench his thirst. On the Hyperion, the crew picks up the TARDIS on radar. Elsewhere a Mogarian, very steampunk before it's time, must switch on his translator to communicate. Crew members flirt. Someone enters the communications room, stuns the crew, and flips some switches. A mayday signal sounds in the TARDIS. They materialize in the Hyperion hold. The Doctor is hesitant, claiming evil is present. He's stopped from leaving by guards who take them to the bridge. Here they meet Commodore Travers, a man who has been helped by The Doctor before. They don't know who sent the mayday, so Travers confines The Doctor to the passenger quarters, planning to use him to flush out the culprit. In the hold there are plant pods. In an office among them, a Mogarian turns on a light and steals some Demeter seeds, like huge silver Mentos. The light hits a plant pod and it begins to glow.
The Doctor and Mel talk in the lounge. Mel is from a place called Peas Pottage, but that's all we learn about her. Mel pushes a worried Doctor to solve the mystery. We see an isolation room that a masked Doland or Bruchner goes in past a guard. In the gym, Lasky is working out and Mel comes in. She dons a headset for workout music, because that's how it's done in gyms. Bruchner, or the other one, comes in and gets Lasky because of a problem at the hydroponics. Mel's music stops and a voice speaks to her. The Doctor is checking the passenger list when Mel comes and tells him to meet whoever sent the mayday in cabin 6. Cabin 6 is in shambles. They find some Demeter seeds and a boot. They run off the the waste disposal where someone has fallen in, or been pushed. There is a boot left that matches the lone boot in cabin 6, which they learn was Grenville's cabin. The Doctor says the case is over, suggesting Mel go investigate the hydroponics center. In trial The Doctor interrupts, not remembering that, claiming those aren't even his words. He feels he is being manipulated and evidence has been distorted. The Valeyard says it's them who are being manipulated, saying The Doctor sent Mel to die. Mel and crewman Edwardes go to the hydroponics to find Lasky. He tells Mel the lights must be low spectrum to keep the plants dormant. He opens the cage and there are huge flashes of light, Mel screams, and boy does this companion scream.
The plants come alive and kill Edwardes. Another guard comes and takes Mel to safety as another guard is killed. The Valeyard claims this is The Doctor's fault, who said it isn't what happened when he viewed the matrix. He says the only way to discover why he's being manipulated is to press on. Laskey and The Doctor are talking in the gym when guards come to take him to the bridge. Travers is questioning Mel, who says there was a booby trap and if not for Edwardes she would be dead. The bodies can't be found, so that clear Mel. Travers laments The Doctor's appearance on thee scene. In the lounge The Doctor and Mel asks Lasky about the Demeter seeds. She accuses him of stealing them and he explains. The Hyperion adjusts it's course. In the lounge the Mogarians are, no kidding, playing Galaga. I could only find a small pic but I'm surely including it. They stop and make complaints individually about going so close to a black hole. They complain about the humans and their taking of Mogar's minerals. The Valyard interrupts as to the point, but The Doctor points out if he had payed attention, he would have known one of them was about to die.
The Mogarians are served drinks as the passengers mill about the lounge. One rises, choking, then collapses to the floor. The Mogarians object when The Doctor removes the ventilator mask, but it is not a Mogarian. He is identified as Grenville, from cabin 6. The Doctor knows him as investigator Hallett, who the old man says he had recognized him as earlier. The Doctor figures that's why he staged his death. When The Valeyard asks how The Doctor knew he wasn't a Mogarian, they show the clip again and see he didn't need to use his translator. He says he couldn't choose to stay on the sidelines. Mel follows The Doctor to the hydroponics, where The Doctor shows her a leaf he found on Hallett. They wonder what came out of the pods. Laskey is talking with her men about their experiment and discovery remaining a secret. The isolation room was just an unfortunate accident. Someone, or something, watches from the air vent. The old man orders a drink from Janet. In his cabin he enters the bathroom and is killed by the sting of something still unseen. It's plantlike and it turns the shower on when Janet returns to fool her. Determined to get in the isolation room, The Doctor sets off an alarm and the guard runs. The Doctor and Mel enter. There is some tank or chamber, and when they open it, the body of a woman lies inside, part of her head some type of strange plant hybrid. Her one good eye opens.
The plant woman begs them to stop Lasky. Lasky and her men come in and shoo The Doctor and Mel out. She was a lab assistant who had a scratch on her thumb during a cross fertilization experiment. They called it a calculated risk. Security comes and takes The Doctor for setting off the false alarm. The Valeyard fails to comprehend the evidence. The Doctor begs the same restraint he showed, and The Valeyard chuckles. The Doctor defends his actions to Travers. Janet comes and tells him the old man is missing. Mel overhears and goes to his room, finding a leaf in the vent. In the hold, the Vervoids are piling bodies. They are some of the worst looking monsters. I can't even begin to describe them without being vulgar. Plant people, leave it at that. Travers agrees to cooperate with The Doctor. Security chief Rudge is forced to sit and talk to the Mogarians. Vervoids attack more crew. Doland sees Bruchner destroying their paperwork. In the gym, Lasky won't tell Mel what the leaf is. Doland comes in and tells her what Bruchner is up to and she leaves. Mel hears voices in the vent and uses her headset to record them. She hears the Vervoids plotting to kill them. Someone grabs Mel. The Doctor looks for Mel, who he doesn't see being wheeled past him in a trash bin.
The Doctor finds the recording, hears Mel scream, puts 2 and 2 together and gets to her just before pulverization. The tape was now missing, though. Mel goes to spy on Janet, The Doctor says he's off to the hydroponics. After a quick scene of a Mogarian clumsily spilling a drink there is a shot of The Doctor, holding an axe, standing in a destroyed communications room. In trial The Doctor objects that he's not the one who did the sabotage, and states the matrix is wrong. When asked if there's any point in continuing, he says what else does he have and they continue. Mel is snooping in Janet's room when a Vervoid comes in. She hides in the shower. Lasky is mad at Bruchner for his destruction of files. He knocks her out and flees. The Doctor finds her and warns her Bruchner has gone wild. On the bridge, Bruchner forces Travers out, locking the door. Bent on destroying the ship he changes course. In Janet's room the Vervoid goes wild, then starts spraying a fog. Outside the bridge, The Doctor tells the others that Bruchner is aiming the Hyperion 3 into the black hole of Tartarus.
Vervoids get into the vents, including the one in Janet's room. Mel escapes the fog. They cut through the door to the bridge, but the Vervoids have fogged the bridge, Bruchner is dead. Rudge calls the Mogarians, who don't breathe air, and they go in and set the new course away from the black hole. Then the Mogarians, with the help of Rudge, take over the ship. They take the group to the lounge. Mel, Doland and Janet stay hidden. Rudge gets the keys to the vault to get the minerals the Mogarians feel were stolen. On the bridge, someone unseen throws a liquid on the Mogarians that kills them. Mel climbs in a vent and talks to The Doctor. He tells her to attack the bridge. On the bridge, they take the masks off the Mogarians to prove to Rudge his plan is over. The Doctor gets Rudge's upside down gun, which Mel thinks is odd for him to want. He also announces a search for the missing audio tape. Doland takes The Doctor to a safe in Laskys office. The safe is locked but Doland had the tape all along. He gets the gun The Doctor has set down to examine the safe. He vows that he will get the Vervoids to Earth to be used as slaves. The Vervoids hear this. But The Doctor had disarmed the gun. Doland runs, to be caught by guards, who all then get killed by Vervoids.
Laskey believes there was a DNA malfunction in the Vervoids. They talk about how they want to kill all animal kind. Co-existence wouldn't work. Travers begs The Doctor's undivided commitment to help. In trial, The Doctor uses this to show he did not interfere, and The Inquisitor agrees. The Valeyard, however, prefers to await the outcome. Ingredients to make an herbicide have been destroyed. Lasky tried to reason with the Vervoids and is killed. A crewman kills a Vervoid, a Vervoid kills a crewman. The Doctor and Mel find the Vervoid's body pile. This upsets Mel. The Doctor tries to calm her by likening it to a compost heap. As they all try to fight off the Vervoids, The Doctor gets the idea of using the metal the Mogarians were after, a mineral that could release intense light and carbon dioxide to accelerate their life cycle. They fake a generator fault and shut down main lighting. The Vervoids retreat. The Doctor and Mel listen to them, but one finds Mel. She screams and releases the mineral. There are bright lights and negative lights. The Vervoids collapse, wither, and die. Thank yous all around and The Doctor and Mel leave. In trial The Doctor admitted that none of the creatures survived. The Valeyard is excited. He now says it is no longer a matter of meddling, but that it was now an issue of breaking article 7 of Gallifreyan law. The Doctor objects. The Valeyard says no exceptions, this was now a case of genocide.
Next: The concluding story for "The Trial of a Timelord": "The Ultimate Foe"