The Malus, from the planet Raaga, came to Earth on board a Harkol probe, and arrived in Little Hodcombe around 1643. The Malus is a psychic being, and is able to amplify emotions in people nearby and feed of the energy.
On the 13th July 1643 the English Civil War came to Little Hodcombe. With all the fighting and emotions the Malus was able to feed off the energy of the locals during the fighting. The locals were able to capture the Malus and seal it in the walls of the church.
In 1984 the Doctor, Tegan and Turlough visit Little Hodcome to see local historian Andrew Verney - Tegan's grandfather. During their time there is a war game going on to re-enact the English Civil War. During this re-enactment the emotions are wake the dormant Malus.
Using the TARDIS the Doctor is able to break the psychic link between the Malus and the local villagers. Being startved of energy the Malus destroys itself, and the church it was sealed inside.