It is the 1930's, and the Doctor is in New York. This is not the last time he will be in New York in the 1930's, which leads to ENDLESS TEARS. But we're not there yet. (I mean, we regularly get endless tears from this series, but that's beside the point.)
There have been some odd disappearances in the city, which are being chalked up to Depression-related suicide. But Solomon, The Wise Old Man running a Hooverville, believes something more sinister is afoot. The Doctor, Martha, Solomon, and this kid named Frank decide to find out by signing up for construction work with Mr. Diagoras, whose name is sinister enough to give you pause.
In the sewers, the Doctor finds goo (sewer goo becomes very important in Series 9, by the way), and then the pig monsters from Amnesia: A Machine for Pigs arrive. (Or, alternatively, the pig men from House on the Borderlands. Pick your flavor of terror.)
Piggy piggy no no piggy piggy GO HOME
Frank is captured, and possibly gets bitten by a radioactive spider afterward, but the others escape into a theater, where they meet Tallulah, who is a very lovely showgirl but has a voice that will crack glass. Tallulah's boyfriend Laszlo was also captured by pig men. The Doctor Macgyvers together a matter analyzer for the goo, and promptly freaks out, while Martha and Tallulah chase a pig man back into the sewers. To no one's surprise, they discover the pig man is Laszlo, and no one who has read the title is surprised to find a Dalek either. The goo was leftover Dalek from an experiment gone wrong.
How much of that goo was down there? Now that we know about what happens to dead Daleks, should we be concerned about a zombie Dalek goo uprising in New York? I'M VERY CONCERNED ABOUT THESE THINGS.
Anyways, the Daleks are using "dumb" humans as pig slaves, and the smarter ones are being set aside for yet another attempt to merge Daleks and humans. It never works, but by Davros they are going to keep trying! Dalek Caan ponders this on the Empire State Building. Fans who saw The Chase giggled into their palms.
"So we pretend The Chase never happened?"
Meanwhile, Dalek Sec drags Mr. Diagoras into his casing and merges with him, giving us this:
Some fans laughed, some fans cried, some were just hypnotized by the way the tentacle dreads NEVER STOPPED TWITCHING.
Evolution of the Daleks
So the plan is to combine Dalek DNA and human DNA, because that always turns out well, between the revolution and the existential crisis and the insanity. All they're waiting for is a gamma strike from a solar flare, because gamma energy totally works that way. Sec is already coming up with less world ending plans, and since the human bodies are already brain dead, the Doctor agrees to help Sec and drop them all off on a world where they can mind their own business, for once.
The other Daleks think this plan is far too sensible, so it's mutiny time! Dalek Caan takes over, and Sec is chained up like a dog, because most aliens' superiority complexes involve equating humans with non-sentient species.
The Doctor decides to climb to the top of the Empire State Building to pull off the Dalekanium panels that will conduct the gamma energy to the Daleks' lab. He doesn't succeed, so remember kids, when you're looking at the Empire State Building, you're looking at pieces of Dalek armor.
Show of hands, how many of you actually tried to climb up there and find out? That's what I thought. Why are you looking at this in prison?
Anyways, the Doctor gets struck by a gamma strike which is totally not lightning.
The Dalek hybrids wake and stand around being paler than a Goth kid in the winter.
Sec tries to talk some sense into the Daleks, because five minutes after absorbing a human he promptly forgets that Daleks are completely irrational. Because he is sane, he dies. Luckily, the Dalek hybrids have also gained some sanity and fight back, and point out that they aren't really Daleks. The Doctor agrees-the Super Magic Gamma Strike somehow infected them with Time Lord DNA. Dalek Caan, realizing that this will mean endless trolling, executes the hybrids, then uses the emergency temporal shift to hop over into the next season finale.
The Lazarus Experiment
You know what fictional characters hate? Immortality serums. Drives 'em up the wall. After their "one" adventure, the Doctor drops Martha back off at her apartment, and they watch a clip of the news about an old guy changing the meaning of being "human". The Doctor leaves, then promptly comes back, because ADD affects Time Lords too.
He and Martha decide to investigate, despite her mother being super suspicious of him, and also slapping him.
He just can't win...
At Professor Lazarus' demonstration, the oldster enters a machine and comes out much younger. He promptly begins flirting with Martha's sister, while Martha and the Doctor examine the DNA sample (from where he kissed Martha's hand earlier) under a microscope.
Turns out he activated some unknown gene, which is revealed when he turns into a scorpion-lizard thing and drains his "partner"'s energy. So, basically, it's straight out of the Lovecraft mythos. (No, seriously, there's a story out there-can't remember the name-where some guy doing experiments devolves into a lizard being.) Our heroes find him on the roof, putting the moves on Martha's sister, though he probably planned on devouring her literally, not figuratively. They all flee the transformed Lazarus, and the Doctor and Martha get locked in his transmogrification box.
I was mainly just looking for an excuse to use this.
The Doctor reverses the polarity, because it's his go-to strategy when all else fails. It sends the box's energy out, seemingly killing Lazarus.
But that would be way too easy! After the ambulance picks him up, he revives and noms the driver, then heads to the cathedral, where he hid during the London blitz. Through some very weird use of physics, the Doctor somehow manages to make the pipe organ hit just the right vibration to mess up Lazarus' fragile DNA, and he dies for reals this time. The Doctor and Martha head out for more adventures, and Martha doesn't get to hear her mother's frantic voicemail warning her about the terribleness that is the Doctor.
It's probably because she heard about this coat.