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#1435
Klisélista :)
Alien Invasions:
Most alien civilizations devote their time and energy to invading other planets. It is not clear why this activity is so popular. Perhaps it's just their way of being friendly.
In particular, these aliens like to invade Earth. We seem to be prime real estate. Even species who are manifestly unsuited to the local environment want it. The reasons are, again, unclear; it's difficult to imagine what we might have that can't be had more easily elsewhere. Ownership of Earth would be a big resource sink, as well, since you'd have to defend it from all the other aliens who want it.
Base Under Siege:
The Doctor arrives at a base. Bases are always located in isolated areas such as alien planets, space stations, or Antarctica. As soon as the TARDIS lands, the base comes under siege from implacable enemies. The Doctor must use his special talents to help the personnel, who are useless in a crisis, defend themselves.
If this is a television episode, the base will feature a large room in which most of the action will take place.
Factions in Space:
A society, usually a human colony in the future, is divided into two factions. Relations between the factions vary from mild unrest to outright war, but in any case the Doctor is caught between them. The situation usually worsens just as he arrives.
Often an external threat just happens to show up at the same time as the Doctor. Evil Aliens work well for this. If so, either the factions will learn to work together to fight the new menace, or escalating tensions will hinder the Doctor's efforts to defeat it.
Fiction Becomes Reality:
Fiction—sometimes a specific work, sometimes a genre—becomes real, through dimensional weirdness, advanced technology, or some other device. The Doctor is thrown into the resulting environment.
This idea hasn't quite become a cliché yet, as writers are still coming up with new takes on the plot every time they use it. However, it's popular enough that cliché status is inevitable.