Overview
The series is set in Cardiff and follows the Welsh branch of a covert agency called the Torchwood Institute which investigates extraterrestrial incidents on Earth and scavenges alien technology for its own use, its origins outlined in the Doctor Who episode "Tooth and Claw". As the opening monologue explains, the organisation is separate from the government, outside the police, and beyond the United Nations. Their public perception is as merely a 'special ops' group. The events of the first series take place some time after the Doctor Who series two finale, in which Torchwood's London headquarters was destroyed. This format was maintained for the first two series.
Series three, a miniseries, saw the Cardiff headquarters completely destroyed and the team temporarily operating as fugitives in England's capital city of London, its membership wiped and the organisation thoroughly broken over the course of the serial. Series four, another mini-series, starts with Torchwood fully disbanded. Jack has left Earth after the events of Series Three, and Gwen has retired to be with her family. The group is reformed (unofficially), this time operating primarily in the United States and including two fugitive ex-CIA agents framed for treason.
Read more about this topic: Torchwood