In the novel Destination Unknown, Walter Griffiths is a friend of the nuclear scientist Thomas Betterton. Shortly before Betterton disappeared, and while he was working at Harwell, Griffiths came over from America to visit him.
According to Betterton's wife Olive, Betterton and Griffiths had known each other well while in America and they seemed pleased to meet each other. Griffiths was U.S. government official who worked in the District Attorney's office. According to Olive, their conversation was mostly local gossip.
The significance of the meeting with Griffiths is only made clear at the novel, when it is revealed that Griffiths told Betterton that there would be an exhumation of Betterton's first wife. This likely panicked Betterton and was one of the trigger events leading to his disappearance.