It's nearly the end of the Second World War. There's a mysterious new code and Alan Turing has been called in to crack it. Everyone assumes it's a German code, as that's where the source is emanating from - apart from Turing's new friend, the Doctor.When people start dying, the Doctor's knowledge of the code and the codemakers casts suspicion onto him, and even Turing starts to have suspicions.
Meanwhile, in an African village, writer and spymaster Graham Greene has found something far stranger than an unusual code.
The trail leads through occupied Germany, across the front line. What they find defies human explanations... but does the Doctor really have all the answers?