The Seventh Doctor and Ace go to the library on Kar-Charrat to return some (very) overdue books on "Juggling For Alpha Centaurians". The library contains every piece of information from every planet: a collection to rival the Gallifreyan Matrix. Because such knowledge could be dangerous in the wrong hands, the library is hidden from most species by some time tweaking. Its planet is wet, covered in jungle and perpetual rain, and the library itself is located at the top of a waterfall.
The library rarely gets visitors these days, which — as Ace points out — might just be because no one knows it exists, and those who do know can't be bothered to climb a waterfall and leap through an undetectable time barrier. The Doctor, meanwhile, checks out the library's new data storage system, which runs on water.
Ace gets bored and wanders back out to the rainy wilderness, where she meets Bev, a treasure hunter whose entire team was wiped out by a mysterious shouty robot, which certainly won't be important later. Bev herself doesn't do much this serial, but she'll be back later.
They get rather rudely interrupted by a dormant Dalek invasion fleet. The Daleks had been waiting for a way to get into the library (for a rather long time, in fact), and Ace wandering off with the key device was just the opportunity they needed. They clone Ace (which doesn't fool the Doctor), get past the library defenses (which doesn't scare the Doctor) and hack into the water-based data (which... kills the Doctor). Fortunately, the local sapient lifeforms turn out to be the rain itself, and they rescue him from the brink of death by absorbing his mind into their collective consciousness. Unfortunately, the data storage system turns out to be made of the sapient rain and practically lobotomised the species, and the Doctor gets really, very angry at the library staff when he finds out.
One of the Daleks gets all knowledge of everything ever uploaded into its brain, and Nicholas Briggs gives his hammiest performance yet when the Dalek goes stark raving mad and subsequently decides to use its power for good instead of evil. It's killed by the other Daleks, of course, and the Doctor saves the day.
From the Daleks' perspective, the first part of the Dalek Empire story. It's also Nicholas Briggs' first ever time officially voicing the Daleks in proper, licenced Doctor Who, a job he holds to this day.
- Body Horror: A very, very rare heroic example, as this is the Kar-Charratans' only way to fight back against the Daleks.
- Breakout Character: The first appearance of Bev, who will reappear in "Dust Breeding" and is all over the Bernice Summerfield stories.
- Call-Back: The Daleks make use of their rarely-seen robot duplicates (as in "The Chase").
- Call-Forward: An unintentional one. The Doctor's mind gets "saved" into the Library's files, akin to what he would later do for River Song in "Silence In The Library" / "Forest of the Dead".
- Evil Me Scares Me: Ace doesn't like her Dalek clone one bit. Neither does the Doctor.
- Doctor: She lacks a certain charm.
- Moral Event Horizon: In-Universe: The Doctor's disgusted repudiation of Elgin on learning of his genocide of the Kar-Charratans.
- My Skull Runneth Over
- Mythology Gag: Bev was named on the basis that a Terry Nation-pastiche story had to have a character with a one-syllable first name and the surname Tarrant. (cf. Jill Tarrant in "Death to the Daleks", Cal Tarrant in the Dalek Annual, Dev, Del and Dieter Tarrant in Blake's 7, etc).
- Out of Time, Out of Mind: The very long-term Dalek plan to take over the Library.
- Puppeteer Parasite: The Kar-Charratans inhabit water, as in they're the water. They can do this to humanoids. Subverted in the fact they're friendly, and don't harbor any ill will towards the Doctor and co..
- Robot Me: The Dalek replicant of Ace.
- Starfish Aliens: The Kar-Charratans are pretty "out there" even for Doctor Who.
- Talking to Himself: Aside from Ace / Robot Ace, there's the several Daleks which only have two voice actors.
- Token Good Teammate: The new lifeform the Daleks create based on the "knowledge of the universe".
- The Voiceless: Prink never manages to get a word in edgewise until moments before his death.
- What Measure Is a Non-Human?