A Nazi spy spent six weeks in a small Donegal village in the hope of turning it into a secret German submarine base ahead of a potential invasion of Britain, a new BBC documentary reports. 

Professor Ludwig Mühlhausen, a German scholar who rose quickly through the ranks of the SS, traveled to Teileann in Southwest Donegal  for six weeks to "improve his Irish."

Locals, however, were always suspicious about Mühlhausen's intentions for visiting the tiny fishing village and BBC journalist Kevin Magee told the compelling story about how the German was a fervent nazi who identified Teileann as a potential backdoor route into Britain. 

Mühlhausen didn't exactly hide his nazism, hanging a gigantic picture of Adolf Hitler on his bedroom wall as soon as he moved in. 

The German scholar and celtologist studied Irish for six weeks in the Donegal Gaeltacht but also took numerous pictures of the village and measured the depth of Donegal Bay. 

The documentary, entitled "Nazi sa Ghaeltacht, documented how the Nazi lowered lead weights into the tide at Donegal Bay, leading locals to believe that he was testing for a potential U-boat submarine base. 

Mühlhausen sent reports back to Germany and described the Irish economy as a "waste" and said that there was a lack of enterprise among the Irish people. 

He claimed in his reports that Ireland would benefit from German efficiency, which could make fishing and farming more profitable. 

Perhaps unsurprisingly, the presence of a fervent Nazi in a tiny Donegal village didn't go unnoticed and the Irish military intelligence kept reports on Mühlhausen, describing the German as "an enthusiastic Nazi." 

"He thinks German culture would be good for us, and Ireland would be better run by Germans or British," one report said. 

Read more: Hitler’s revenge on an Irish village - three girls died

The Irish military archives additionally contain files on the Nazi plan to invade Ireland, which appear to include images and notes taken by Mühlhausen. 

Ludwig Mühlhausen left Ireland after six weeks, never to return to the country, but he would return across Irish airwaves as a Nazi propagandist, addressing locals in the Irish language. 

He recited British atrocities in Ireland in the Irish language and encouraged Irish people to remain impartial or support Germany during the war. Incredibly, Mühlhausen also reminisced about his time in Teileann during the broadcasts. 

Mühlhausen's Irish story didn't end there, however. 

A prisoner of war in Naples, he sent a letter to then Irish President Douglas Hyde, who he viewed as a personal friend, in an old Irish script appealing for help.

There is no evidence to suggest that Hyde ever replied to the appeal. 

And so ended this incredible story of an ardent Nazi's unusual relationship with Ireland. 

Kevin Magee summed up the story on Sunday night's documentary. 

"The plot reads like a World War two thriller, except this story is for real," he said. 

Read more: The unlucky internees during Ireland's WWII "Emergency"

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