You can read the first two portions of this three part essay here and here.
After listing these primary reforms that Lycurgus brought to the Spartans, Plutarch shifts gears, indulging in a somewhat exhaustive survey of the social institutions and practices that set Sparta apart from its neighbors. While this is all presented as “ideas introduced by Lycurgus,” the multi-generational aspects of his analysis of those traditions turns this middle portion of the book into something other than merely a biography on Lycurgus. Step-by-step, we are taken on a little tour of classical Sparta from its education systems (for boys), its war-making machine (for older boys), its law-making practices (you get the drift).
Though this section of the work is interesting, it also muddles up our sense of time flowing as Lycurgus, a man who lived and died, is lost to the traditions that were born from his reorganization of Spartan culture. There is an implication throughout the work that Lycurgus (or the Senate or some combination of the two) instituted all of the reforms in a relatively short period of time and yet the picture that Plutarch sketches out for us looks more like a time-lapsed photo than a snapshot in time. Much like Pythagoras (with whom Lycurgus’ legacy shares many commonalities), traditions that would have taken generations to develop into actual mores are attributed to the iconic law-giver with little attempt to harmonize the inconsistencies. This cognitive dissonance is amplified when Plutarch returns to the actual subject of his narrative.
When he perceived that his more important institutions had taken root in the minds of his countrymen, that custom had rendered them familiar and easy, that his commonwealth was now grown up and able to go alone…[Lycurgus] conceived the thought to make it immortal too, and, as far as human forecast could reach, to deliver it down unchangeable to posterity. 
Calling a great assembly, Lycurgus tells his people that he has one final edict to issue but before he does, he wishes to consult the Oracle at Delphi one last time. He elicits an oath from them to change no law until his return and, with that, makes his way to the Oracle. His question for the Oracle was simply whether his reforms had been for the good of Sparta. When the Oracle returned an answer that his law were sound and would result in the ascendancy of Sparta for many generations to come, Lycurgus sprung his secret plan into action. Committing the Oracle’s response to writing, he sent it to the people of Sparta and never returned, thus binding them to their oath to uphold his laws without change essentially forever. He, then, undergoes a slow and stately suicide by starvation at an undisclosed location.
So what are we to make of this biography of Lycurgus? The entire account is historically challenged by Plutarch’s accurate description of his subject in the section’s heading as legendary. Indeed, the Sparta that deposed tyranny throughout Greece for centuries and opposed Athens’ attempt to dominate the Mediterranean was itself mostly legend by the time Plutarch is writing in the first century of the Common Era. Still, Plutarch enjoys certain advantages over others who might attempt a similar work in his sustained access to the Delphic records which spanned the epoch that separates him from his subject. In combining three sources (what was commonly believed about Lycurgus, what was known about Sparta, and the records of Lycurgus’ consultations at the Oracle), Plutarch is able to assemble a story that straddles history and myth; taking from both what best harmonizes with his own authorial agenda while discarding that which does not.