Season fifteen kicks off where season fourteen finished: Gothic horror. The TARDIS materialises in a lighthouse on an isolated piece of shoreline just after a mysterious light has been seen falling from the sky and one of the lighthouse-keepers is found electrocuted.
Soon, a sudden fog bank descends, causing a ship to founder on the rocks. The survivors straggle ashore but they soon find that they are locked in the lighthouse with a Rutan - a jelly-like alien that can take other forms and has been masquerading as lighthouse-keeper Reuben. The Doctor kills the Rutan warrior with a makeshift explosive, and then uses a diamond that one of the survivors had been carrying to focus the lighthouse's light into a laser beam to destroy the incoming Rutan mothership.
The Rutans are described by the Doctor as being the ancient and implacable enemies of the Sontarans, who have appeared multiple times in the show's history; this is the only appearance so far of the Rutans.
- Blue Blood: Lord Palmerdale and company. The Doctor bottles up his contempt for them.
- Closed Circle
- Cue the Sun: The story ends while dawn is breaking.
- Death by Materialism: The last surviving non-regular character, Skinsale, is killed after his greed gets the better of him and he scrabbles for the diamonds that the Doctor had thrown away (only needing the best one to focus a laser). The Rutan gets him. It had been established earlier that greed was his Fatal Flaw.
- Death Ray: The Doctor using his MacGyvering skills to weaponize the lighthouse beam into a sort of Archimedes Heat Ray to blow the Rutan mothership into smithereens.
- Dissonant Serenity: How the Doctor breaks the news of the lighthouse attack to Harker, Palmerdale, and Skinsale, complete with a grin:
The Doctor: Gentlemen, I've got news for you. This lighthouse is under attack and by morning we might all be dead. Anyone interested?
- Doomy Dooms of Doom: This episode was originally going to be called "Rocks of Doom."
- The Edwardian Era
- Electric Jellyfish: The Rutan is pretty much a space version of this.
- Everybody's Dead, Dave: The Doctor and Leela are the only survivors, everyone else is massacred.
- Face Stealer: the Rutan dissects the first two lighthouse operators in order to learn enough human physiology to take the form of Reuben for the final two episodes. Later victims it just kills.
- Fainting: the Emotional Faint, pulled by Adelaide on hearing of Lord Palmerdale's death.
- Fog of Doom: caused by the Rutan scout trying to adapt the local environment to its own needs.
- Ominous Fog: For atmosphere.
- Get a Hold of Yourself, Man!: Arguably when Leela slapped Adelaide.
- Insignificant Little Blue Planet: the Doctor is quite surprised by the Rutan's interest in Earth. It turns out to be fairly mild.
The Doctor: Why invade an obscure planet like Earth? It's of no value to you.
- Inspired By/Very Loosely Based on a True Story: The Flannan Isles mystery proved the inspiration for this adventure, the poem is even quoted by The Doctor.
- Jerkass: Lord Palmerdale
- Kill'Em All: No one survives this one except for the Doctor and Leela.
- Lighthouse Point: Where all the action takes place.
- Mineral MacGuffin
- The Radio Dies First: Skinsale sabotages the wireless to thwart Palmerdale's schemes.
- Real Life Writes the Plot: Companion Leela's eyes changed from brown to blue as a result of the Doctor's laser, but the real reason was that actor Louise Jameson had decided to stop wearing the brown contact lenses.
- Screaming Woman: Adelaide...until Leela slaps her.
- Sickly Green Glow: The Rutan.
- The Siege: A "base under siege" story like many before it.
- Spoonerism: Leela consistently mangles technological terms, from "boiler pressure" to "Mognal Sigulator" (signal modulator).
- Temporary Blindness: Leela
- Voluntary Shapeshifting: the Rutan, with great effort.
- The X of Y