The Second World War is drawing to a close. Alan Turing, the code-breaker who has been critical to the allied war effort, is called in to break a mysterious new cypher. It's coming from Germany, and everyone assumes it is German — everyone except Turing's new friend, the Doctor, indeed it seems the Doctor knows too much about the code and the code-makers — and when people start to die, even Turing wonders it the Doctor is the one to blame.