The Ne PluS Ultra line was based on a system of fortresses built by Vauban. Its claim to fame was that it "extended" from the English Channel to the Swiss border, but it was only about two (fortresses) deep.
The line was not a cure-fall for the French defensive system. Basically, the fortresses were the hinges of the "line," which was only as good as the ability of the French to cover the gaps between the fortresses. Another answer discussed how Marlborough was able to "pick off" an underdefended fortress at Ariex, near Arras. When the French army moved to repair this breach, Marlborough went for his real objective, the strongest fortress of Bouchain, at the heart of the line, and on the direct road to Paris.
Both sides had about 90,000 men, but Marlborough detached 30,000 to conduct the siege (against 5,000 defenders), so his covering force of 60,000 was outnnumbered 3 to 2 by the French. Not only did he defeat attempts at relief, but he strengthened his supply line going back to Belgium. When the siege was over, there was only one fortress between Marlborough and Paris. This victory is less heralded than others because it involved less than 10,000 French casualties, but Winston Churchill (a descendant of Marlborough's), considered it his "finest" victory, because it was won at a location, and on terms and conditions of the French choice.
Like the Maginot Line, the Ne Plus Ultra Line was "porous" because the British army outmarched the French, first at Arieux, then at Bouchain. (Wellington won battles with similar tactics in Spain.) In "A War to be Won" Professor Williamson Murray attributes the German victory in France in 1940 to the fact that the German Army (and armor) went on "long training marches" in the winter of 1939-40 and the French did not. Basically, Marlborough's penetration of the Ne Plus Ultra Line was an "attack through the Ardennes," so it's unfortunate that it is so poorly understood.