Guy Hamilton, best known for his work on Bond films such as 'Goldfinger' (1964) and 'Diamonds Are Forever' (1971), directs this adaptation of the story by Edgar Wallace. The Ringer is a feared and enigmatic criminal. Though believed by some to have died in Australia, a series of threatening notes received in London indicate to police that the Ringer is nearby - bad news for corrupt lawyer Maurice Meister (Herbert Lom), the subject of the threats, who the Ringer holds responsible for the death of his sister. A police unit is dispatched to Meister's house to offer protection. Police psychiatrist Dr Lomond (Donald Wolfit) is part of the team in attendance and tries to help Meister stay calm in the face of the threat. However, as it becomes increasingly clear that the Ringer has infiltrated the team and is within striking distance of his target, the composure of all concerned will be firmly tested.