Zig-Zag was the codename of Eddie Chapman, a criminal imprisoned in Jersey for safe-breaking[?] in Glasgow. On the occupation of the Channel Islands, the Germans recruited him as an agent and despatched him to mainland Britain to commit acts of sabotage. He was turned and was used to feed misinformation to the Germans.
... complete the square[?]. To "complete the square" is to add a constant (i.e., in this case, a quantity that does not depend on x) to the expression to the left of "=", ...