Cracking a Killer's Code, Pt. 1

S1, E1: Susan, noticing a pattern in a string of London murders, contacts her old friends from Bletchley — Millie, Lucy and Jean — to try to identify the killer. Treating the crimes like a code to be cracked, the women gather all available information and evidence and then, through a sequence of clever deductions, determine how the killer is finding his victims and where a missing girl must have been abducted. Can they reach her before the killer claims his fifth victim?