A Foreign Legionnaire comes to ask Gladys for help in locating the grave of a fallen colleague's mother so that they can be buried together, and Trevor is not thrilled when the man – Rheiner, a handsome German – starts to show an interest in Cecile.

Gladys is busy giving evidence in court, so Trevor starts making enquiries. However when he talks to Eavis he learns that the policeman believes Rheiner's story to be a fake, and that there is a connection to a jewel robbery and the untimely death of one of the thieves. However the Moons and Cecile agree to join forces with Rheiner in order to recover the missing diamonds for the insurance money.

Eavis learns that Rheiner is a murderer and warns Trevor and Cecile, who arrange for Zelda to conduct a séance at which she 'contacts' the dead Legionnaire. As a result of this the diamonds are located in an engine shed on the Romney, Hythe and Dymchurch Railway, although Trevor manages to substitute fakes which are then stolen at gunpoint by Rh