"The Unicorn and the Wasp" is the seventh episode of the fourth series of the revival Doctor Who. It stars David Tennant as The Tenth Doctor and Catherine Tate as Donna Noble.
After having some dramatic, daunting and galaxy spanning adventures, The Doctor and Donna find themselves back on earth. More specifically, they find themselves in the Roaring 20s, at a posh party where the guest of honor is Agatha Christie. They barely have time to soak in the ambience before The Doctor realizes that they have shown up just as Agatha Christie has had her (factual, and still unsolved) 10 day disappearance. And then, a visitor, Professor Peach, is murdered in the library. And since this is an episode of Doctor Who, it is not just a earthy murder mystery, but involves an alien. It is up to the Doctor, Donna and Agatha Christie to solve the mystery together.
This is what is termed a "bottle episode", with few uses of special effects. There is also not any timey-wimeyness, and no threats of universal destruction. The episode instead depends on the ensemble cast and character interaction, which at times can be quite slapstick. By some standards, this could be seen as the weakest episode of Series Four, since it doesn't develop the character arc. But even if someone were to take that view, it is still quite good. The murder mystery is genuinely mysterious, the actors and characters all interact well, and the 1920's are portrayed in detail. It says something about how good this Series is that even its seemingly minor episodes are done with so much quality.
"How is 'Harvey Wallbanger' one word?!