Captain Jack Harkness is a time traveller originally from the 51st Century. He first appeared in Doctor Who as a con man. After helping the Doctor fight the Daleks, being killed and then being brought back to life by Rose Tyler, Jack transported back to the 19th Century and set up the Torchwood Organization. Outside the government, beyond the police, Torchwood protect Cardiff from alien threats from a secret underground base. Jack first met Gwen at a crime scene, after which he recruited her to join the team. From the very beginning, there were hints that Jack and Gwen may have romantic feelings for each other.

Gwen Cooper was a PC at a crime scene when she was first introduced to the Torchwood team. She was intrigued by them, especially the untraceable Captain Jack Harkness. When she finally tracked down the base, Jack saved her life and asked her to join the team. While she's been there, Gwen has been the only person who Jack has fully trusted. 

Jack and Gwen have been in many dangerous situations together. On Gwen's first day as part of the Torchwood team, Jack saved her life and she rewarded him with a kiss. Jack has often been a shoulder for Gwen to cry on, and she has often been his pillar of strength. When Jack left the team at the end of season one to find the Doctor, Gwen was obviously badly affected. Upon his return, she seemed a little unwilling to trust him. Jack appeared to be devastated when he finds out that Gwen is engaged to boyfriend Rhys, and she seems to be pointing the finger at Jack, suggesting that if he had not left, she would not be engaged. In Something Borrowed, during a conversation with who she thinks is Jack, Gwen leans in to kiss him dressed in her wedding dress, waiting to marry Rhys. However, it is not Jack but an alien Shapeshifter, and Gwen marries Rhys in the end. Although Gwen's marriage seems to be the end for Jack and Gwen, a lot of people seem to have high hopes that there will be more between them in the future.