Alan Jackson gets a job in the United States and Maria must prepare to leave her friends on Bannerman Road behind. This episode marks Yasmin Paige's last appearance as a regular.
Kaagh, the lone surviving Sontaran from "The Sontaran Stratagem" plots revenge against the Earth by dropping satellites onto nuclear reactors.
Tropes:
- Attack Its Weak Point: Chrissie attacks Kaagh's probic vent. With her high heels.
- Badass Boast: An unconventional one but Sarah Jane successfully intimidates Kaagh by displaying her otherworldly knowledge of the Sontarans' history and future from her past encounters with his kind. It convinces him that she is truly an "extraordinary female".
- Batman Grabs a Gun: Knowing all too well the potential threat even a single Sontaran can pose, Sarah Jane instantly decides to contact UNIT to help deal with the situation in spite her usual preference to leave trigger-happy militarised organisations out of her lighthearted adventures. However, she did call UNIT before back in Revenge of the Slitheen after the local slitheen cell was dealt with.
- Call-Back: Stating "Some people never learn", Chrissie breaks into Sarah Jane's house through the same window she did in "Eye of the Gorgon".
- Chekhov's Classroom: Chrissie walks in right when Mr. Smith is telling Alan how to defeat a Sontaran. See below for how that turns out.
- Combat Stilettos: Chrissie jams a high-heeled shoe into Kaagh's probic vent.
- Continuity Nod: To "The Sontaran Strategem" in the main Doctor Who series.
- As Maria enters the attic to take one last look around and to say goodbye to Mr. Smith, she looks to her left and sees numerous alien objects Sarah Jane has collected, including a bottle of Bubble Shock! seen in "Invasion of the Bane", Mr. Smith's Portable Scanner, a Sontaran gas canister and Kaagh's gun taken from Kaagh in this story, the MITRE headset seen in "The Lost Boy", a puzzle box seen in "Whatever Happened to Sarah Jane?", and entanglement shells seen in "Warriors of Kudlak".
- Kaagh's motivation is being able to avenge his fleet's defeat from "The Poison Sky".
- Not to mention all the way back to Classic Who stories "The Time Warrior" and "The Sontaran Experiment", which also featured solo Sontaran reconnaissance units as antagonists. Sarah Jane directly references her previous encounters with Sontarans in these stories to intimidate Kaagh.
- Famed In-Story: Part of Kaagh's motivation for destroying Earth's defences is so that he can finally return to Sontar as a war hero rather than a disgraced remnant of a failed invasion.
- I Don't Like the Sound of That Place: The village with the observatory is called "Goblin's Copse."
- I Was Beaten by a Girl: After his plan is foiled, Kaagh can do nothing but lament the unbearable humiliation of being defeated by "a female and half-forms". Too ashamed to return to Sontar, Kaagh decides to become a space-faring mercenary.
- Innocently Insensitive: In response to Clyde's joke, Maria tells him that it won't be the same without him. He replies that at least he isn't going anywhere, which clearly upsets Maria.
- Internal Reveal: Chrissie discovers Mr. Smith, and Alan tells her about Sarah Jane and the kids having adventures with aliens.
- Laser-Guided Amnesia: Subverted. Chrissie is knocked unconscious after hitting Kaagh in his probic vent and the gang just assumes she'll wake up with no memory of what happened (or at least be convinced it was all a dream). At the end of the episode, Chrissie reveals that she remembers everything.
- Look Behind You: Maria says, "Look! UNIT!" to a Sontaran. However, the Sontaran claims he looked deliberately as he knew where Maria and the others would escape to, and it would save him having to chase them.
- Oh, Crap!: Sarah Jane's reaction to the Sontaran ship is one of pure dread. She instantly decides to contact UNIT even though her companions believed that they should always be discreet and peaceful with their encounters.
- Put on a Bus: Maria and Alan.
- The Remnant: Kaagh is the last survivor of the Tenth Sontaran Battle Fleet and his sole motivation is to destroy all life on Earth out of spite.
- Series Continuity Error: Maria claims Sarah Jane has never wanted to call in UNIT before, but she did in the end of Revenge of the Slitheen. Maria even correctly guessed she was calling in friends from UNIT. She also claims she has an anti-military stance in general, when this was never mentioned in the series before.
- Sequel Episode: Kaagh is out to destroy the Earth in retaliation for the events of "The Sontaran Stratagem"/"The Poison Sky".
- Shout-Out:
- When Sarah Jane asks Mr. Smith if he has acquired a sense of humor since his reboot, he replies, "I will run a diagnostics check immediately" whilst playing the sound effect associated with the Book from the television adaptation of The Hitchhiker's Guide to the Galaxy (1981).
- When Alan tries to convince Chrissie that Maria is playing an alternate reality game, Chrissie compares it to the time they spent "looking for a golden rabbit" when they were dating, referring to Masquerade.
- You Watch Too Much X: Clyde claims Kaagh watches too many Conan DVDs after Kaagh brags about his titles.
