Have you never seen a single episode of Dr. Who?
If so this list of seven great episodes may help you, from revision3.com

In no patricular order (info from imdb.com)

The Waters of Mars (2009 November 15)
In a Mars base the inhabitants are being infected by a mysterious water creature which takes over its victims. The Doctor is thrust into the middle of this catastrophe knowing a larger one is waiting around the corner.

The Impossible Astronaut (2011 Apr 23) Season 6 Episode 1
River Song, Rory, The Doctor and Amy are reunited in the Utah desert

Doomsday (2006 July 08) Season 2 Episode 13
Earth becomes the battlefield for the greatest and deadliest war of all time, as the Daleks and the Cybermen clash with the whole universe at stake. The Doctor and Rose, reunited with old friends and Cybermen experts Mickey and Jake, race to find a way to bring the war to an end before it brings about the destruction of the whole of space and time. But the Doctor soon faces an even bigger dilemma – could ending the war mean the death of Rose?

Human Nature (2007 May 6) Season 3 Episode 8
In 1913, Martha watches in jealousy from afar as The Doctor learns what it is to be human and to fall in love with the local school nurse, Joan Redfern.

The Eleventh Hour (2010 April 03) Season5 Episode 1
With his TARDIS in ruins, the newly-regenerated Doctor with the help of Amy Pond must save the world in less than twenty minutes from galactic policemen known as the Atraxi.

Blink (2007 June 09) Season3 Episode 10
“Don’t Blink. Blink and you’re dead. Don’t turn your back. Don’t look away. And don’t Blink. Good Luck.” These cryptic messages left on 17 DVD’s leave Sally Sparrow on a journey to assist the Doctor and Martha Jones, who are trapped in 1969. But Sally is in 2007, and they won’t meet until 2008. Strange? Unusual? Not if you are the Doctor.

The Rebel Flesh (2011 May 21) Season 6 Episode 5
The Doctor, Amy and Rory arrive in a future Earth where a small group of humans operate a plant extracting highly dangerous chemicals. To minimize the danger to themselves, they create clones made of a plastic-like substance, ‘gangers’ (short for doppelganger) who do the actual work. While forming their clones however, the Earth is struck by massive solar flare activity that transfer the humans’ emotions and memories in addition to their technical abilities