In the episode, a killer shape-changing alien known as Arcateenian called Mary (Daniela Denby-Ashe) has been stranded in Cardiff for nearly two hundred years.

Torchwood is called to a building site where a centuries-old human skeleton and a rusted alien artefact have been discovered in the ground.

The next day, Toshiko asks Mary her true identity but she remains coy, answering "Philoctetes", the name of an archer from Greek mythology who was exiled on an island.

Toshiko wears the pendant in a crowded street, and though overwhelmed with the thoughts of everyone, identifies one man preparing to commit murder, and she is able to follow and stop him.

Meanwhile, Owen has discovered that the same trauma that had been inflicted on the skeleton has been reported several times in the last few centuries, while Jack becomes aware of Toshiko's strange behaviour.

Destrii's home planet Oblivion along with Devos, Krant and Arcateen IV, V and VI form the Arcan system.