Captain Jack Harkness, the former Time Agent and con man from the 51st century last seen traveling with the Doctor, ventures to early 21st century Cardiff. There, he becomes a member of Torchwood Institute, a renegade criminal investigation group founded by Queen Victoria to battle hostile extraterrestrial and supernatural threats.

"Torchwood" is an anagram of "Doctor Who". When the first series of "Doctor Who" (2005) was being made, television pirates were desperate to acquire the preview tapes. One of the people in the office had the idea of labeling the tapes with the anagram "Torchwood" rather than "Doctor Who", as a security measure to disguise the tapes when they were delivered from Cardiff to London. Writer Russell T. Davies liked this idea so much that it later inspired him to use it as a title when creating this spin-off series.

Captain Jack Harkness:
[voiceover during first season opening] Torchwood: outside the government, beyond the police. Tracking down alien life on Earth, arming the human race against the future. The twenty-first century is when everything changes. And you gotta be ready.