In 2008, Torchwood Three discovered a sentient element in the water of Cardiff Bay. It looked and smelled like oil, but had a different chemical composition to any known element on the 21st century periodic table. Toshiko Sato observed that this compound element sought to replicate itself, and moved in ways that displayed basic intelligence and possible conscious thought.

As well as assimilating any other elements it came into contact with, breaking them down and making them part of itself, this element was able to take on the physical form of individuals, such as Ianto Jones, or of large structures such as a ship called The Dread. While it could recreate physical form fairly accurately, the element was unable to render distinct objects. All of the items aboard The Dread, for instance, appeared to be fused to the walls of the ship. The element was also incapable of producing any audible sounds. Its replicas of humans were almost-perfect, but could be spotted by their oily-black and lifeless eyes.

The element sought to spread itself as far and wide as possible. It travelled to Cardiff through the Rift as part of its plan to replicate itself across space and time. Upon arrival, it immediately took the form of a boat, and upon reaching the shore began moving towards a fire engine, the first vehicle it could find. As well as being drawn to vehicles, the element sought to extend itself by breaching power-supplies and replicating along electrical networks, something Ianto and Toshiko both observed.

The sentient element was quickly eradicated in Cardiff when a coastguard plane flew over the bay, spreading a trail of a petroleum-based bio-remedial agent, which contained micro-organisms that could eat through the hydrocarbons of the petroleum-like compound, reducing it to water. (PROSE: Black Water)