Series writer and creator Anthony Horowitz also writes a book series in which a fictionalized version of himself teams up with a private investigator to solve crimes. The opening chapter of The Sentence is Death finds Horowitz on the set of The Eternity Ring, witnessing the difficulties around filming the street scene where Sam gets off the bus. Horowitz acknowledges the anachronistic bus (see Goofs section).
The character "Aleksei Gorin" is loosely based on the Soviet defector Igor Gouzenko.
The location for the external shots of the MI5 HQ in this season was the Old Customs House in Dublin, Ireland.
The pub where Foyle meets Hoffman and Tomasz is Walsh's in Stoneybatter, Dublin.