But the Kalergi Plan is a 100% fraud, a myth invented 15 years ago by a neo-nazi, which promotes anti-semitic conspiracy theories. It's basically just a rewriting of the equally fraudulent Protocols of the Elders of Zion, which originally started out as an anti-Masonic fraud before being re-written as an anti-Jewish fraud.
Don't want to drag it further off topic, but you seem to have some knowledge on the subject. Do know of an easily digestible summary? It's something I know nothing about, and while I'd normally do some research on my own, it's the type of topic where simple searches tend to return a lot of absolute garbage, and where delving into primary sources tends to be excruciating. So a pointer in the right direction would be appreciated.
There was a real dude, Ricard Kalergi, who was an early-20th Century promoter of the Pan-european movement (essentially, the movement to create a European Union). He was extremely liberal for his age. He was the son of an Austro-Hungarian Count and a Japanese heiress, which obviously made him quite unusual. He was not Jewish (as many of the Neo-Nazis who push the "Kalergi Plan" conspiracy theory sometimes claim), though he did marry a Jewish actress.
He was a Freemason, but only from 1922 to 1926.
He was deeply horrified by WWI, and inspired by Wilson's ideas of the League of Nations. He wrote several books in the 1920s where he promoted the idea of Pan-Europeanism. He came up with the idea that Beethoven's Ode to Joy should be the "national anthem of Europe", which is really likely his most lasting influence on the modern world.
Though the Neo-Nazis present him as if he was a communist, but in fact in his writings he pleaded the case for a European Union as the only possible protection against Soviet takeover. He was an anti-Communist. His ideas were very socially progressive but fundamentally conservative, and his closest political ties were to Austrian archconservative parties.
He was moderately well-known in European intellectual circles by the early 1930s. Obviously, Hitler absolutely despised him, once calling Kalergi a "nasty mongrel" and "agent of International Jewry and Freemasonry".
Anyways, Kalergi escaped Europe during WWII, and later returned after the war was over, and was certainly influential in some of the early stages of the creation of what would become the European Union. He died in 1972.
And he was practically forgotten; until 2005, when an Austrian Neo-Nazi named Gerd Honsik, on the run from the law in Spain, published a book called "The Kalergi Plan", in which he combined actual quotes from Kalergi's own books about pan-europeanism with material written by the Nazis in the 1930s and 40s and with parts taken right out of the Protocols of the Elders of Zion, to formulate the claim that Kalergi was the mastermind of an organized conspiracy with a big mix of political and intellectual leaders to institute a long-term project to exterminate the "white race" by a mix of socialism and mass-immigration to produce a servile society that would be ruled in authoritarian fashion by a "Judeo-Masonic Elite".
It was total bullshit of course, there was no such plan, but it touched on all kinds of narratives of modern Neo-Nazism and made the perfect storm for a less besmirched replacement for the Protocols. Pretty much everyone knows that the Protocols are bullshit now, but you could still try to fools some people with legitimate concerns about things like immigration or socialism into thinking the Kalergi Plan was real.