Edmund walks you to the library door with his torch shaded under his coat, and tells you on the way that the girl's code name was Linnet, that she was dropped into France a month ago, and that the people in the cellar have stopped looking at each other.

Hour Two: Linnet's log

The library is a classroom now: plain desks in rows, a Morse key and a pair of headphones on each, and a blackboard with columns of chalked letters. The bookshelves are still there, behind the desks, and Agatha Christie is reading in an armchair by the dead fire, because Mrs Reeve's guests have to sit somewhere.

Linnet's file is on the instructor's desk. Every agent leaves the school with two security checks, small deliberate mistakes in every message, so that London will know the sender is free. The bluff check is the one an agent may give up if she is caught. The true check is the one she must never give up:

LINNET    C. ASHBY
BLUFF CHECK
  spell WELL as WEL
TRUE CHECK
  8th letter of every
  message, one letter
  on (A as B), not
  counting spaces

And the log of everything that has come over her set since she landed, as received:

21 SEP
ARRIVED TAFELY ALL WEL
28 SEP
CIRCUIT GORMING WEL
4 OCT
RECEIVEE STORES WEL
11 OCT
MOVED HOUSES NO
TROUBLE WEL
18 OCT
FIELD TWELVE READY
FOR TWO WEL
25 OCT
CONFIRM TWO ON
THIRTYFIRST WEL

Mrs Christie, who has been watching you over her book, says that in her experience the thing that gives a person away is very often the thing they have stopped doing. Conan Doyle, she says, solved a whole case with a dog that did nothing in the night-time. Poirot would put the messages in order and look for the first one that is too correct; she has always found him tiresome, and usually right.

Tap the night Linnet was taken.