Carlie Roberts was a marine geologist, specialising in the Mariana Trench.
Carlie originally worked at Torchwood One, working with Ianto Jones. Though Carlie was interested in Ianto, he was with Lisa Hallett at the time and didn't act on her feelings. After the fall of Torchwood she was stationed in Tokyo.
In 2009 she was investigating a cry for help in the Mariana Trench. Ianto Jones contacted her from Torchwood Three and requested her assistance. Together they were able to get the USS Calvin and the Octopus Rock from UNIT to investigate. When they reached the trench, they were attacked by Sam Doyle, a human who had been infected by an alien parasite and who made the cry. The parasite fed on memories and wanted another human for a host. After the Octopus Rock was scuttled, Carlie and Ianto stayed in it while Gwen Cooper and Jack Harkness went to investigate the Guernica a previously crashed submarine. Though they were able to escape onto the Guernica, the parasite followed them and infected Carlie. Using her body, the parasite tried to kill Gwen, but it died when the submarine continued to rise and separated it from the hydrogen sulphide it needed to survive. Though Carlie was almost killed in the process, Ianto was able to save her with CPR. Following the adventure, Carlie planned to make further expeditions into the Mariana Trench to find out what else was there. (AUDIO: Submission)