On the night of May 10, 1941, a Scottish farmer named David McLean found a German Messerschmitt airplane ablaze in his field and a parachutist who identified himself as Captain Alfred Horn. McLean's mum was soon serving him a cup of tea by the cottage fireside, but their surprise guest was no ordinary Luftwaffe pilot. Incredibly, he was Rudolf Hess, a longtime Hitler loyalist, to say the least. Hess joined the Nazi party in 1920, stood with his friend Adolf Hitler at the Beer Hall Putsch, and served in Landsberg prison -- where he took dictation for much of Mein Kampf. As deputy Fuhrer, Hess was positioned behind only Hermann Goering in the succession hierarchy of the Nazi regime that had Europe firmly under the heel of its jackboot.

Hess's appearance on Scottish soil, a self-described mission of peace just weeks before Hitler would launch his ill-fated invasion of the Soviet Union, was one of the war's strangest incidents. The search for explanations began on the morning after and has roiled on now for 75 years, spawning theories both intriguing (World War II might have ended differently) and bizarre (the man wasn't Hess at all but a body double.) The truth is likely as interesting as any of the fantasies—but it's still not entirely certain what happened 75 years ago.

The Hess flight was remarkable in itself. He left an airfield near Munich in a small Messerschmitt fighter-bomber a little before 6 p.m., flying up the Rhine and across the North Sea. Hess displayed considerable skill by navigating such a course alone, using only charts and maps, on a foggy dark night over largely unfamiliar terrain—all while avoiding being shot down by British air defenses. By 10:30, Hess was over Scotland, out of fuel, and forced to bail out just 12 miles from his destination.

That unlikely site was Dungavel House, home of the Duke of Hamilton. Hess hoped to make contact with one of the highly placed British figures who, unlike Churchill, were willing to make peace with the Nazis on Hitler's terms. Hess believed that Hamilton headed a faction of such people and immediately asked his captors to be taken to him. But Hess was misinformed. Hamilton, who wasn't home that night but on duty commanding an RAF air base, was committed to his country and to its fight against Germany.

The unlikely envoy's mission quickly took a turn for the worse. When granted a meeting with Hamilton the next day Hess's pleas fell on deaf ears. Worse for Hess, he denied from the start that Hitler knew anything of his mission, which meant that the British afforded him none of the diplomatic respect to which he thought he'd be entitled. Instead he was imprisoned, and by the night of June 16, the obvious failure of his mission left Hess so mentally shattered that he attempted suicide by hurling himself down a flight of stairs.

Hess spent the war in British hands, confined in various locales including (briefly) the Tower of London and a military hospital at which he was even allowed guarded drives in the country. He was visited frequently by intelligence officers eager for secrets and by psychiatrists eager to plumb the Nazi mind—which in Hess's case increasingly showed serious signs of mental illness. The psychiatric examinations were rooted less in concern for Hess's mental health than in the hope that this fanatically devoted Nazi could provide them valuable insights about how the criminals ruling Germany, including Hitler himself, thought.

Hess was transferred back to Nuremberg for the post-war trials in October, 1945, where he escaped the hangman but was sentenced to life in prison. He spent the rest of his long life, 46 years, as Prisoner Number 7 in Spandau where he lingered long after the other Nazis were freed. Hess was the facility's only prisoner for more than 20 years, his term ending only when the 93-year-old was found hanging from a lamp cord in a garden building in August 1987. The suicide was denounced as a murder by those, including Hess's own son, who suspected he'd been silenced.

But Hess's death didn't end the questions. Had he really come alone? Had someone sent him to Scotland or had someone sent for him?

News of Hess's flight was a bombshell in Berlin, and Nazi authorities quickly moved to disassociate him from the regime. The German public was quickly told that Hess suffered from mental disturbance and hallucinations.

Joseph Goebbels, the Nazi propagandist who knew much about such tactics, feared that the British would use Hess as part of a devastating campaign targeting German morale. He worried in his private diary on May 14 that the German public was “rightly asking how such a fool could be second to the Fuhrer.”

But the furor gradually died down. Though Hess held a powerful title, his actual influence in the Nazi hierarchy had waned dramatically by 1941, so much so that some have speculated that his flight was born of hopes to regain Hitler's favor by delivering him an agreement with the British. Instead his departure simply consolidated the power of his ambitious and manipulative former deputy Martin Bormann.

Yet a persistent theory has suggested that Hess's ill-fated peace mission was actually carried out with Hitler's knowledge—and the understanding that he'd be disavowed as insane if it failed.

In 2011, Matthias Uhl of the German Historical Institute Moscow unearthed some purported evidence for this claim. Hess's adjutant, Karlheinz Pintsch, had handed Hitler an explanatory letter from Hess on the morning after the flight, and Uhl discovered a report featuring Pintsch's description of that encounter in the State Archive of the Russian Federation.

Pintsch claimed that the Hitler received his report calmly. The flight occurred "by prior arrangement with the English,” Pintsch wrote, adding that Hess was tasked to "use all means at his disposal to achieve, if not a German military alliance with England against Russia, at least the neutralization of England."

This version aligns well with Soviet claims dating back to Stalin himself that British intelligence services had been touch with Hess and duped him into the flight. In fact they may align too well, for the statement was produced during the decade when Pintsch was an often-tortured Soviet prisoner and its language smacks of Cold War propaganda terminology—suggesting the Soviets coerced the version from Pintsch.

Indeed other witnesses reported a very different reaction from Hitler. Inner circle Nazi Albert Speer, waiting outside Hitler's office during the meeting, described the Nazi leader's reaction as “an inarticulate, almost animal out-cry” of rage. “What bothered him was that Churchill might use the incident to pretend to Germany's allies that Hitler was extending a peace feeler,” Speer wrote in Inside the Third Reich. “'Who will believe me when I say that Hess did not fly there in my name, that the whole thing is not some sort of intrigue behind the backs of my allies? Japan might even alter her policy because of this,'” he quotes Hitler, while also noting Hitler's hope that Hess might luckily crash and die in the North Sea.

Speer discussed the flight with Hess himself 25 years later when both were incarcerated in Spandau. “Hess assured me in all seriousness that the idea had been inspired in him in a dream by supernatural forces,” he said. "We will guarantee England her empire; in return she will give us a free hand in Europe." That was the message he took to England— without managing to deliver it. It had also been one of Hitler's recurrent formulas before and occasionally even during the war.”

British historian Peter Padfield explores the “British duped Hess” theory in Hess, Hitler & Churchill. As with much of the Hess affair definitive evidence is lacking but a few tantalizing possibilities exist. Padfield has unearthed intriguing nuggets from period sources: the diary of a well-placed Czech exile who'd viewed a report suggesting an English trap, reports of Soviet spies who'd uncovered now untraceable evidence of the same. In 2010 the son of a Finnish intelligence agent who'd been on Britain's payroll claimed that his father was involved in the plot.

The official records that have been made available, perhaps not surprisingly, reveal no such role for the British intelligence services. The most plausible motivation for such a plot, were it ever to have existed, was that the British hoped it would convince Hitler to scrap or at least postpone an invasion of Britain; a peace settlement would make such a drastic and dangerous step unnecessary and free him to focus on the battle against his most hated enemy—the Soviet Union.

MI5 files declassified in 2004 suggest that Hess did have his adviser Albrecht Haushofer pen a letter to Hamilton in 1940, suggesting that a neutral site meeting could advance secret peace talks. British intelligence intercepted that letter, investigated (and exonerated) Hamilton for being part of a pro-peace Nazi plot, and seriously considered the possibility of replying to set up a double-cross.

But they dismissed the scheme and simply let the matter drop without ever knowing that Hess was the man behind the communication, the official files suggest.

However those files are far from complete. Some of the intelligence files on the Hess affair are known to have been 'weeded,' or destroyed. Whatever information they held is lost—but other classified files remain and have yet to be released.

Earlier this week, the Duke of Hamilton's son, James Douglas-Hamilton, called for the British government to release its remaining classified documents concerning the affair.

Conspiracy theorists suspect that the documents could contain not only transcripts of interrogations but correspondence between Hess and other figures including George VI. But Douglas-Hamilton, who has written his own book on the Hess affair, suspects they won't embarrass prominent Britons who really did want to deal with Hess but rather they'll likely confirm the standard story.

“The evidence shows Britain had an honorable record in fighting the Third Reich and did not swerve from that position,” he told The Scotsman. “Excessive secrecy with regard to the release of relevant material has, and can serve to, obscure that reality.”

In recent years a few other secret files have emerged. In 2013 a U.S. auction house offered an astounding folder of documents, still marked top secret, some 300 pages that appear to have been authored by Hess himself during his wartime captivity and carried with him to the Trial of the Major War Criminals in Nuremberg. They had been missing ever since.

The files are shrouded in a Hollywood-style intrigue; who got their hands on them, and how exactly, and why did they then simply give them away to the current seller for nothing via an anonymous phone call? But the papers themselves tend to dispel mysteries rather than raise them, and that’s assuming that the contents are genuine. The auction house made some scans and transcripts of them public for the sale, and it’s unclear if they ever changed hands. In one of the digitized documents, Hess described his interview with Hamilton on the morning after his flight in a passage that perhaps provides the best window into the workings of the mind that conceived this unusual attempt.

“The British cannot continue the war without coming to terms with Germany…By my coming to England, the British Government can now declare that they are able to have talks…convinced that the offer by the Fuhrer is genuine,” the files note.

But the rulers of Great Britain were convinced of no such thing. Former Foreign Secretary Lord Simon, the highest-placed person known to have met Hess, interviewed him on June 10 a few days before his first suicide attempt. "Hess has come on his own initiative,” Simon wrote of the meeting. “He has not flown over on the orders, or with the permission or previous knowledge, of Hitler. It is a venture of his own.”

With that Hess was simply locked up for the rest of his long days, though Winston Churchill, writing in The Grand Alliance, claimed at least some distress at his fate.

“Whatever may be the moral guilt of a German who stood near to Hitler, Hess had, in my view, atoned for this by his completely devoted and frantic deed of lunatic benevolence,” he wrote. “He came to us of his own free will, and, though without authority, had something of the quality of an envoy. He was a medical and not a criminal case, and should be so regarded.”

RELATED: During his captivity Hess often suspected that his meals were being poisoned. Incredibly, food packets that he wrapped and sealed at Nuremberg for future analysis have been sitting in a Maryland basement for 70 years.