8. Last Of The Time Lords
Last of the Time Lords served not only as the climax to NuWho Series 3, but also as the last regular appearance of companion Martha Jones. She bids the Doctor goodbye at the end of this episode, leaving Number Ten alone once again. Poor fella.
However, he has no time to mope around, because the bow of a ship called Titanic crashes through the walls of the TARDIS. The Doctor's expression of bewilderment says it all. He even says "What?!" right after this happens.
This collision of worlds was to set up the Christmas special Voyage of the Damned, which saw the Doctor team up with Kylie Minogue to escape a doomed space liner, also named Titanic.
You'd think they would've learned from the first time.
It's safe to say that nobody had "crashes into the Titanic" on their Doctor Who bingo card that year, but this bizarre ending helped cut through the sadness of losing Martha, and created much intrigue for the upcoming holiday episode.