St Matilda's College, Oxford is haunted. The building was formerly a convent and, so the story goes, three ghostly nuns wander its passages during the hours of darkness. The story goes on to say that anyone who sees the 'three sisters' will not be long for this world. The most brilliant IT brain in the country can't use her computer. More worrying, the exhibits are attacking the visitors, while outside, people seem to be losing control of the technology that runs their lives. When one of the students mysteriously disappears, the Dean of the College, Dame Emily Shaw, has no option but to call in the police. Her call appears to be answered when a Police Box arrives in her study; the Doctor and Leela have come to investigate and uncover the dark secret that has lain buried beneath the college for almost a thousand years...