Trouble again finds Esther Brookstone on her home turf. An old friend from her Oxford days is developing encoding and decoding algorithms that involve entangled quantum states and quantum computers, all motivated by some of Leonardo Da Vinci's ideas found in a newly discovered notebook of the artist. His project is financed by MI5, but both the Americans and Russians want his results...and so does a mysterious stranger. Cloak-and-dagger suspense abound in this fifth novel of the series.