Rosalyn Landor

Holmes is approached by Helen Stoner, who feels threatened by her domineering step-father, Dr Grimesby Roylott. Her sister died mysteriously just before her planned wedding, and Helen has heard again the curious whistle she first heard on the night of her sister's death. Holmes perceives great danger and lays a complicated plan to defeat the threat to the beautiful Helen.

The crew's rescue of a missing earth colony leads to the discovery of a civilization composed entirely of clones.