Summary: Sarah Jane, Clyde, and Rani meet Ruby, another alien fighter.
Sarah Jane, Clyde, and Rani head to the crash site of a germ-carrying meteor. However, a mysterious woman disinfects it before they can.
The gang returns to Bannerman Road, where Sarah Jane forgets to put on her car's handbrake, causing it to roll before Clyde stops it. They later find that the strange woman has moved into a house on Bannerman Road. Suspicious, Sarah Jane & Co. confront her and find out that her name is Ruby White. Sarah Jane tries to make peace with her but Ruby brushes her off.
Later, as Sarah Jane returns home from the store, she stops to talk to Gita and says that she needs to make tea for Luke. However, Luke had not returned from university. Concerned about Sarah Jane's memory lapses, Rani and Clyde follow her into her attic. Rani suggests that Sarah Jane take a vacation, but she refuses out of fear that aliens would invade while she was away. Mr. Smith then detects a fleet of Dark Horde battleships headed for Earth. Detecting the alien technology in Sarah Jane's attic, the Dark Horde teleports a team there but Mr. Smith manages to deflect them into a nearby junkyard. Sarah Jane gives Rani a round device meant to overload their sensors and, to the shock of her companions, gives Clyde a blaster.
They go to the junkyard only to find that the sensor overload device requires the sonic lipstick to activate. Sarah Jane had meant to give Clyde the sonic, but accidentally gave him the uncharged blaster instead. Three Dark Horde soldiers advance on them and Sarah Jane realizes that she had gotten them all killed. At the last moment, Ruby appears and throws two similar overload devices to Clyde and Rani. They show the Dark Horde soldiers an illusion of stronger versions of themselves. Frightened, the soldiers return to their ships and the fleet retreats. Ruby apologizes for her attitude at their last meeting and introduces them to her handheld AI, which Clyde dubs "Mr. White."
Sarah Jane takes Ruby back to Bannerman Road and lets her see the attic. Sarah Jane talks to Ruby about her past. But when she starts, she forgets the Doctor's name for a moment. She orders Mr. Smith to perform a medical scan on her. When he finds signs of brain tissue deterioration, Sarah Jane realizes that she's no longer fit to defend the Earth, much less lead Rani and Clyde in doing so.
Ruby becomes fast friends with Rani and Clyde, even teaching Rani how to drive her sports car, making Sarah Jane feel left out. Sarah Jane invites Ruby into the attic and offers her the house and everything in it. Ruby accepts and Sarah Jane gives her command of Mr. Smith, removing her voice from his files. Now firmly in control, Ruby teleports Sarah Jane into a cellar, which contains a giant red organ that Ruby calls her stomach. Ruby explains that she is a Qetesh, a creature that feeds off thrills and emotions. She targeted Sarah Jane because of her exciting life, which she found highly appetizing. Ruby reveals that she set up the diseased meteor and the Dark Horde attack, which were just Mr. White's elaborate holograms. Ruby then draws out Sarah Jane's life and begins feeding it directly into her stomach.
Mr. Smith, under Ruby's control, plays a video for Clyde and Rani which appears to be Sarah Jane telling them she is leaving and having Ruby take over. Clyde reacts poorly as Sarah Jane's sudden unexplained departure reminds him of his father. When Rani defends Ruby, Clyde blames her for giving Sarah Jane the idea of leaving. Clyde then sends Luke a message saying that his mother had run off. Rani goes home where her mother comforts her. Clyde goes into the attic, hoping that Mr. Smith will help him find Sarah Jane. However, Mr. Smith is unable to follow his commands and only gives him a coded message "Beware Ruby."
Ruby finds out that Clyde is suspicious of her and teleports him into a small spherical room in orbit around the Earth. She explains that the sphere was originally a prison cell. The Qetesh exiled her for being unable to control her hunger, leaving her only a video game system. She reprogrammed the game system into the AI Mr. White, turning her prison into a ship. She returns to Earth, leaving Clyde on the ship, which will soon run out of air. Clyde leaves one last message on his phone for his friends.
Having gotten Clyde's message, Luke goes to Rani's house and demands to know what is happening. K9, still in Luke's dorm room, tracks Clyde into space. K9 says that he can reset Mr. White, turning him back into a game system, freeing Mr. Smith and rescuing Clyde. However, he requires a cell phone near Mr. White for it to work. Rani takes her phone into the attic, where she casually sets it down near Mr. White and pretends to have a friendly chat with Ruby. After a few minutes, K9 manages to reset Mr. White, which returns Clyde to Earth and frees Mr. Smith from Ruby's control. Mr. Smith traps Ruby in a containment field and tracks Sarah Jane to Ruby's cellar. Clyde and Rani go to the cellar to rescue Sarah Jane. Ruby breaks out of the containment field and corners them, threatening to digest them directly into her stomach. Luke stops her and gives her a chance to surrender. When she refuses, Luke uses Mr. White's holograms to show meteors falling all over the Earth. The inrush of emotion overloads Ruby and causes her stomach to collapse. Sarah Jane sends her back to her prison cell, which she then sends back off into space.
- Always Someone Better: Ruby's evil plan invoke it.
- Big Damn Heroes: Luke. Also invoked by Ruby.
- Cannot Spit It Out: Clyde mentions to Rani in a message he's recording that he "always...always..." before collapsing from lack of air.
- Cardboard Prison: Ruby was imprisoned in a tiny spaceship by her people, but was allowed to keep an extremely high-tech games console for entertainment... which she used to take control of the craft and escape at her leisure.
- Color Motif: Ruby is often seen wearing red outfits, having a red front door and driving a red car, befitting her name.
- Continuity Nod: Ruby claims that she came to the "Ealing Triangle" because of all the weird things that have appeared there before, including Judoon, the Bubbleshock factory, alien plants, and Trueman.
- The phrase "Mr. Smith I need you" comes from Barbara Smith's note to Eddie Smith, i.e. Sarah Jane's parents. When Clyde comes up with "Mr. White", Ruby says it reminds her of her dad.
- Covered in Gunge: Double Subversion. At the climax, Ruby's (separate) stomach swells to immense size, and Clyde predicts it's about to "splurge." It doesn't, and instead implodes. He walks over to inspect the remains — and then it splurges him.
- Emotion Eater: As a Qetesh, Ruby has a voracious appetite for all "exciting" emotions, especially fear, but her hunger was so uncontrollable that she was banished by her own kind. Observing that life on Bannerman Road is never dull, she decides to take Sarah Jane's place, draining her in the process.
- Everything Is An I Pod In The Future: Mr. White.
- Evil Counterpart: Ruby is introduced as Sarah Jane's younger and more efficient counterpart in alien-fighting, but she is later revealed to be more than she seems.
- Interrupted Declaration of Love: Clyde is trapped on a spaceship running out of air. He records a video for his friends and, addressing Rani, says "I always...always..." before he loses consciousness.
- My Skull Runneth Over: Ruby cannot handle the sudden inrush of emotions and thrills from six billion people.
- No Endor Holocaust: Our heroes defeat Ruby White by projecting a hologram of a meteor shower all over the world to overwhelm her with the human race's panic. The fact that this would logically result in crashes and heart attacks is never mentioned. We could assume that it was worth it to stop her from becoming powerful enough to destroy the world, but no one even brings it up.
- Ominous Latin Chanting: The crashing meteor and the encounter with the Dark Horde.
- Red Herring: A Gareth Roberts episode, with Sarah Jane's name in the title, and a plot where events lead to her considering giving up the alien-hunting business? We all know who the villain's going to turn out to be here, right? Wrong.
- Shadow Archetype: In many ways, Ruby reflects what Sarah Jane was like at the beginning of the series: a cold, aloof thrillseeker, free of all attachments.
- Super Strength: Ruby has superhuman strength, as demonstrated when she was able to crush a mobile phone in her hand.
- Suspiciously Similar Substitute: Ruby villainously attempts to become this for Sarah Jane.
- We Will Meet Again: Ruby vows revenge as her ship leaves Earth.
- Younger and Hipper: Ruby has cooler gadgets than Sarah Jane, most notably a portable supercomputer named "Mr. White" (although only after she takes a liking to Mr. Smith's name), which has a distinct Apple product aesthetic. She even has a cooler car.