During the Second World War, a white line was painted around the boundary of the Vatican. It was done on the orders of Oberstrumbannführer Herbert Kappler, head of the Gestapo in Rome.
Kappler ran his network of spies, informants and double agents from the Via Tasso. He arrested anti-fascists, Allied servicemen and Resistance fighters and, in a sound-proof room, tried to make them talk. By 1943, he had become preoccupied to the point of obsession with an Irish priest who, Kappler was convinced, was helping escaped Allied servicemen.
Monsignor Hugh O'Flaherty, who worked in the Vatican's Holy Office, was Kappler's target. He had O'Flaherty watched, followed and, on several occasions, tried to have him kidnapped and killed.
Hitler had assured Pope Pius XII that the Vatican's borders would be respected. Kappler could not simply raid O'Flaherty's rooms in the German College and arrest him. So he had the white border painted to convey a simple message: step over that line and I'll have you.
The Vatican line divided two men who could not have been more different. O'Flaherty, who was 46 in 1944, was a tall, genial Kerryman, popular on the diplomatic circuit and a scratch golfer. Born in Cork, this son of an RIC constable grew up in violent times: seminary friends were killed in the First World War and during the War of Independence he was picked up and questioned by the Black and Tans.
But after spells in Rome and abroad, O'Flaherty found himself on the Vatican's fast track, and was made a monsignor in his mid-30s.
On the other side of the line, watching O'Flaherty as he said his office on the steps of St Peter's, stood Kappler, to all appearances a typical Aryan: blond, blue-eyed and sporting a three-inch duelling scar on his cheek.
Kappler had helped in the interrogation of Georg Ensler, who had tried to assassinate Hitler in November 1939, and also investigated a second such plot, this time by British agents, also in 1939. He was then posted to Rome as a police-liaison officer.
At first, O'Flaherty was a disinterested observer of the war (indeed his Irish nationalism disposed him to be unsympathetic to the British cause), but working as a Vatican envoy to the thousands of Allied prisoners of war affected him deeply. He would return from his camp visits and pass on messages from prisoners via Vatican Radio so that their families would know they were safe.
In the autumn of 1942, an escaped British serviceman made his way to the Vatican, where the British envoy to the Holy See, Sir D'Arcy Osborne, was granted permission to shelter him. By late 1943 Osborne and O'Flaherty had established a fledgling escape line, with a network of safe houses, messengers and informants.
In all, the Vatican Escape Line helped between 4,000 and 6,000 Allied servicemen. They were billeted with families all over Rome and in farmhouses outside the city.
O'Flaherty also helped many Roman Jews, hiding them in Vatican colleges all over the city. Major Sam Derry, who administered the escape operation, thought he was too trusting, but if O'Flaherty was naive, he was also brave, daring and, most important of all, lucky.
On one of his visits to Palazzo Doria, home of Prince Filippo Doria, the place was surrounded by SS troops and capture seemed inevitable. Running to the basement, O'Flaherty noticed that coal was being delivered through a chute. He borrowed some overalls, blacked his face, grabbed a bag of coal, climbed up the chute and was driven away in the coal truck. Kappler, who was waiting outside, was furious.
The escape line survived betrayal and raids, and as the Allies invaded Italy, more and more downed airmen and escaped soldiers made their way to the Monsignor for help.
At the end of the war, O'Flaherty was showered in honours and decorations; Kappler was tried for war crimes. But their relationship didn't end with the German surrender.
Kappler wrote to O'Flaherty from prison, and the Monsignor began to correspond with him and to visit him. Some 10 years later, O'Flaherty baptised his old enemy into the Catholic faith.
There was a further twist to the story: Kappler escaped from a military hospital with the help of his wife in 1977 and was spirited into Germany. He died there the following year.
The story of O'Flaherty and Kappler's game of cat-and-mouse has been told before. The Vatican Pimpernel by Brian Fleming (2009) and The Scarlet Pimpernel of the Vatican by JP Gallagher (1969) are both thrilling and competent accounts.
Hide And Seek, written by the BBC correspondent Stephen Walker, brings new focus to Kappler's side of the story. Using newly available archives in Washington, London and Germany, Walker balances the story and manages to evoke some sympathy for a man who, it seems, really was "just following orders".
The writing is sometimes repetitious, yet the material is so rich and thrilling in itself that it would take a leaden hand indeed to rob it of its drama. Walker does not fall into that trap, capturing well the chaos of the Third Reich in retreat.
He has introduced a new audience to Monsignor Hugh O'Flaherty, noting at the end that there is no memorial in his own country to this quiet hero who was so readily honoured by the British, American and Italian governments.