Martin Trueman is a con-artist who works as an astrologer. One night, while doing a session for a woman named Cheryl, he is struck by a strange light. His astrological powers become real and he takes control of Cheryl, making her his assistant.
Sarah Jane, Rani, Clyde, Luke, Haresh, and Gita all go to see a show being put on by Trueman. He is able to take control of Clyde's actions and accurately reads the pasts of Rani and Sarah Jane, including a mention of the Doctor. He then foretells that Sarah Jane will enter a battle that she cannot win. After the show, Trueman is asked to appear on a television show about the paranormal after the intended guest fell down the stairs in her home (though she lived in a bungalow).
The next day, Sarah Jane and Rani go to interview Trueman, intending to use the scanner watch to see if he is truly human. Not only does he show up as human, he knows that he is being scanned. He then chases Sarah Jane and Rani from his home by casting bolts of lightning from his hands, shouting that the "Ancient Lights" would destroy them.
Trueman abducts Clyde and brainwashes him, ordering him to kill Sarah Jane.
Sarah Jane and Rani return to the attic, where Luke and Mr. Smith have been studying astrology from all over the universe. They theorize that the "Ancient Lights" may have come from a universe from before the Big Bang where the laws of physics were different and astrology worked. The zodiacs from all those different planets were a form of memory that survived the Big Bang. Mr. Smith then suggests that Trueman's astrological birth chart provided the perfect conditions for the Ancient Lights to take him over.
Mr. Smith then detects a broadcast of Paranormal Planet, which Trueman is appearing on. Trueman has caused the broadcast to appear on every television channel. He and Cheryl use their powers to knock out the hostess and crew. Sarah Jane, Luke, and Rani try to leave the attic, only to be stopped by the brainwashed Clyde, who now has powers similar to Trueman.
While Sarah Jane tries to talk Clyde out of attacking them, Gita watches Trueman's broadcast and is put under his control. Trueman explains that all the stars in the universe are in the perfect arrangement for the return of the Ancient Lights. He then begins taking control of everyone in the world according to their signs, starting with Geminis like Gita. They are told to begin joining together in circles.
Sarah Jane tells Clyde to try and kill her, certain that he cannot. Luke puts his hand on Clyde's shoulder. At that moment, Clyde is freed from Trueman's control. Sarah Jane concludes that by forcing Clyde to do something that he couldn't, she freed him. She then decides to try this on Trueman. Rani also suggests that they cut off the theater's power.
Gita and the other hypnotized people go to the New Theater, where Trueman is broadcasting from. Some of them join hands in a circle around the theater. Pretending to still be brainwashed, Clyde bluffs their way into the theater. While Luke and Rani search for the theater's power supply, Sarah Jane tries to appeal to Trueman's human nature. However, Trueman claims that his human side enjoys what he is doing. He intends to create a portal for the Ancient Lights to take over Earth, which they will use as a starting point to take over the rest of the universe.
Rani and Luke find the power switch but the Ancient Lights protect it with a force field. Rani and Sarah Jane are eventually possessed. Only Luke, who does not have a star sign, is left free. He destroys Trueman's zodiac stage display, which severs the Ancient Lights' connection with Earth but causes the theater to collapse. Trueman refuses to escape, not wanting to return to his old life. He then disappears in a trail of gold dust, becoming one with the stars.
- Astrologer: Martin Trueman is an astrologer, whose study of the stars brings him into contact with the Ancient Lights.
- Broken Record:Mr. Smith: Nothing is happening. Nothing is happening. Nothing is happening. Nothing is happening. Nothing is happening. Nothing is happening.
- Con Man: Trueman is a cynical, disillusioned and bitter man who doesn't believe in astrology, yet pretends to be knowledgeable on the subject in order to trick people out of their money.
- Continuity Nod: There are mentions of the Doctor and flashbacks to "School Reunion".
- Deadline News: While describing people getting possessed throughout the world, the newsreader gets possessed. "I accept the Ancient Lights."
- Even Evil Has Standards: Trueman refuses to accept Cheryl's mortgage when he learns she has been using it to pay for her sessions with him, leading him to confess his deception to her in shame.
- Evil Is Hammy: Trueman
- Face–Heel Turn: Clyde. Eventually everyone who isn't a Taurus or Luke.
- For Want of a Nail: The Ancient Lights are a power that survived the Big Bang. They're defeated by the fact that Luke wasn't born and therefore doesn't have a zodiac sign to be controlled.
- From Nobody to Nightmare: Trueman went from phony astrologer to centre of the universe.Sarah Jane: Trueman, listen to me. There must still be something human inside you. Think of your life before this happened.Trueman: My life before? Which part of my life? The part when I was selling vacuum cleaners? When I was rounding up shopping trolleys? When I was cleaning toilets? I was a nobody.Sarah Jane: And this is your way of becoming a somebody? Yesterday nobody cared, today, everybody knows your name. What do you want to be known for, enslaving millions of people?
- A God Am I: That's what "apotheosis" means. Becoming a god.
- "I Know You're in There Somewhere" Fight: Subverted. Sarah Jane thinks this is how she stopped Clyde from killing her. However, it was actually because Luke touched his shoulder.
- Kneel Before Zod
- Let Us Never Speak of This Again: Trueman manages to make Clyde stand on cue, much to his embarrassment.Nobody ever mentions that again, okay?
- Light Is Not Good: And how.
- Madness Mantra: At the end of the first part when Clyde is shaking the house, Mr. Smith keeps repeating "Nothing is happening", because he's incapable of comprehending the forces at work and it almost makes him sound scared out of his mind.
- Mythology Gag: Trueman telling Cheryl that she's "going on a long journey — a very long journey" in the pre-title sequence is an in-joke reference to "Dimensions in Time", in which the Rani utters an almost identical line.
- No Man of Woman Born: A very Macbeth-like example. Trueman is using ancient astrological magic to mind-control everyone in the world one star-sign of birth at a time. Pity for him that Luke is an artificial human being who was never "born" in the usual way.
- Outside-Context Problem: Trueman can control people with astrology using something called the Ancient Lights, despite it supposedly being impossible — even Mr. Smith is unable to deal with it since astrology breaks the laws of physics. Finally, the gang theorise that the Ancient Lights come from a universe that predates ours, one where the laws of physics are different and astrology worked. And then Luke, who wasn't born and therefore has no astrological sign to allow the Ancient Lights to control him, breaks Trueman's plan, essentially making him an Outside Context Problem to the Outside Context Problem.
- Ship Sinking: Martin Truman confirms in "Secrets of the Stars" that Sarah Jane wasn't in a romance with the Doctor.
- Shut Up, Kirk!: "Do you really think that is going to work? Making an appeal to my better nature? Don't you see, the human part of me is loving every moment of this".
- Smug Snake: Trueman
- Suspiciously Similar Substitute: The Ancient Lights were originally intended to be the Mandragora Helix, an Eldritch Abomination Energy Being that Sarah once encountered while travelling with the Doctor, in the story "The Masque of Mandragora". This trope was then enforced when executives decided that the original plan would be too obscure a continuity reference.
- The Watson: Cheryl