The Time Lord and the Ponds get mixed up in a Wild West adventure as the Doctor becomes the reluctant sheriff of a town being terrorized by a relentless cyborg who goes by the name of the Gunslinger. But who is he and what does he want? It seems the answer lies with a mysterious alien whose initial appearance hides a dark secret.

original airdate--September 15,2012       rating--8.42 million/33.4% share #9

music--theme song "The Chase" by Murray Gold

1. "Birdie Song" by The Tweets
2. "Titanium (feat. Sia)" by David Guetta & Sia

Main enemy--Kahler-Tek, Kahler-Jex 

Main setting--Mercy, Nevada, 1870

Goofs:When the Marshall is killed by the gunslinger the doctor pins the badge to his jacket, between shots the badge moves position several times.

• The Doctor grabs a gun from somebody's holster, and we hear him cock it. In the next shot, it isn't cocked and we see him cock it. In the next shot - again - it isn't cocked, and we see him cock it a third time.

Trivia:Scott Bakula was first offered Issac but dates clashed with other projects.

• Not the first Doctor Who to be shot in Spain, the first was The Two Doctors.

*UK rating figures