The Writing on the Wall

First printed in Ellery Queen's Mystery Magazine August 1978. Reprinted in the anthology Ellery Queen's Circumstantial Evidence, edited by Ellery Queen, 1980.
Hugh Pierce was asked by another agency to help crack a coded message while he and his wife, Selena Mead, were vacationing in England. Since Mead was especially good with such things, he asks her to try her hand. Her results will end up saving the life of the American Vice President.