Sarah was a red-haired woman with a necklace capable of jumping dimensions (and possibly through time). She and Tim used it to escape their home dimension's destruction and ran into Richard Llewellyn and Thomas Meaney.
Meaney tried to confiscate the necklace on behalf of the Lead Service but the pair simply transported elsewhere. Sarah and Tim later returned with a missing poster featuring the Leads, but the pair had already left in the Flurry by the time Sarah and Tim came back.
Sarah is named after a lady named Sarah that story writer Benjamin Maydon once knew, and with very little description in the script actually holds a remarkable physical similarity to her.