In the 17th century, Lord Marchwood employed Erasmus Darkening, an alien trapped on Earth, to make gold out of base metals.
In the present, Professor Rivers and her assistant Toby perform a scientific investigation of ghost sightings at Lord Marchwood's manor.
A mysterious poltergeist-like entity begins manipulating objects, including writing the message "get out" on a mirror; this is Lord Marchwood trying to scare everyone off to avoid a similar fate.
Aided by Lord Marchwood's sword as a conductor, Sarah Jane comes up with a plan to stop Erasmus by turning him into electricity.
Pearson Education published a simplified novelisation of this episode by Trevor Baxendale under the title The Haunted House for school literacy programs in September 2010.