"The Girl In The Fireplace" was an incredibly strong episode, right up there with "Tooth and Claw", containing one of the most bittersweet endings so far.
Spoiler:
Premise involves a faulty spaceship, it's occupants are stalking the uncrowned 18th century Queen of France Madame de Pompadour at assorted times via time portals installed in the ship and the Madame's palace. The Doctor enconters the Madame at a young age and saves her from one of her stalkers, a clockwork cyborg race disguised in 19th century masquerade garments. The Doctor keeps making random visits, months and years passing for the Madame in the process, when investigating the time portals back and fourth figuring out why the aliens want her. Mere minutes pass for The Doctor when doing this, as opposed to Pompadour.
Over these visits, The Doctor and the Madame become closer, Pompadour becoming more infa tuated with him, and understanding him very quickly and clearly. Of course, a good old mind meld with The Doctor does that to someone not easily frightened.
It soon transpires that when she becomes thirty-seven, she will become the brain of the powerful alien vessel, also of thirty-seven years in that timeline, and needs a brain of thar age to operate it. The Doctor saves the day, and the aliens cease to function without a way back. The Doctor returns to one of the time portals and promises to take the Madame stargazing, but the saddest part of the episode then kicks into gear.
As The Doctor makes a breif run to assure Rose and Mickey he's fine, he comes back to the 18th century only to find that more years have passed, and the Madame has died of overworked stress, and a heart still waiting for her "lonley Doctor". The Doctor, saddened, departs the vessel, revealed at the conclusion to be the "SS Madame de Pompadour"
A tremendously powerful, and poigant time-crossed love story that never gives the viewer the ending that cannot hope to be acheived despite their yearning it so badly. The circumstances do not allow it, and it displays that adequently
Next week: Alternate worlds, and The Cybermen