Opening Passage: Because this has an elaborate narrative framing, one has to choose what counts as an 'opening passage', since there are two distinct introductions and a Chapter 1 that is explicitly labeled 'Preliminary', with the actual story starting in Chapter 2. Since I don't think the introductions or the preliminary first chapter actually give much a sense of the story itself, I've chosen to begin with the opening of Chapter 2, which at least puts us at the time of the story.
Under the reign of the last Stewarts, there was an anxious wish on the part of government to counteract, by every means in their power, the strict or puritanical spirit which had been the chief characteristic of the republican government, and to revive those feudal institutions which united the vassal to the liege lord, and both to the crown. Frequent musters and assemblies of the people, both for military exercise and for sports and pastimes, were appointed by authority. The interference, in the latter case, was impolitic, to say the least; for, as usual on such occasions, the consciences which were at first only scrupulous, became confirmed in their opinions, instead of giving way to the terrors of authority; and the youth of both sexes, to whom the pipe and tabor in England, or the bagpipe in Scotland, would have been in themselves an irresistible temptation, were enabled to set them at defiance, from the proud consciousness that they were, at the same time, resisting an act of council. To compel men to dance and be merry by authority, has rarely succeeded even on board of slave-ships, where it was formerly sometimes attempted by way of inducing the wretched captives to agitate their limbs and restore the circulation, during the few minutes they were permitted to enjoy the fresh air upon deck. The rigour of the strict Calvinists increased, in proportion to the wishes of the government that it should be relaxed....
Summary: In 1679, Lady Margaret Bellenden holds a wapenshaw (literally: weapon-show), which is a gathering of people in a district "both for military exercise and for sports and pastimes", partly in order to express her support of the Royalists. The Covenanters in the area, of whom there are many, are unhappy, but many of them cannot stay away from the fun and games. During the shooting competitions, two people particularly stand out: Lord Evandale and Henry Morton. The entire story will consist of their intersecting and intertwining lines, and involve three things in particular: they are both in their own way honorable men even when honor is detrimental to their situation; they are both in love with the same woman, Edith Bellenden, Lady Margaret's daughter; and, despite being both political moderates, are on opposite sides of a brewing civil war, since Lord Evandale is a Royalist and Henry Morton is a Convenanter.
Henry ends up getting an immense amount of trouble when his sense of honor leads him to help John Balfour of Burley, a historical figure who played a significant role in the Covenanter rebellion. Balfour of Burley is persona non grata, but he also is responsible for having saved the life of Henry's father, a debt that his father had never been in a position to repay, and so out of a sense of honor, he gives Burley food and shelter for a night. However, Burley was thoroughly involved in the assassination of Archbishop James Sharp, and the result is that Henry's honorable payment of his father's debt leads to near-disaster for his entire family, to Henry himself being outlawed. He ends up with the Convenanter rebels, with whom he has some abstract agreements of principle, being Presbyterian himself and in favor of freedom of religion.
Henry's talents, which were already on display in the wapenshaw, lead him to becoming a significant commander in the Covenanter army, which gains a great victory against the Royalist forces at the Battle of Drumclog. However, it's more than slightly difficult to be a liberalish moderate commander in an army that gets its numbers largely from religious fanaticism; he will be blamed for every failure and mistake that gets made, and despite his genuine commitment to the cause, will constantly be attacked for lukewarmness and Erastian tendencies. It's unlikely that anyone in the seventeenth century would espouse quite the combination of principles Henry Morton does, so it seems fair to say that Henry Morton is a deliberate anachronism, being a sort of pro-Union Scotsman from Scott's own day born back in Convenanter days. He serves as a sort of filter for nineteenth-century Scottish thinkers looking back at the their heritage and having to deal with the problem that a very significant portion of their ancestors would have been vehemently opposed to everything that Scotland had become by the nineteenth-century. Henry is a nineteenth-century test for seventeenth-century Scotland, showing what can be admired in the latter even among the things that the former has repudiated. Nonetheless, he is not an allegorical figure, and is depicted in a well rounded way. Henry understands his allies as to several fundamental principles, but he often shows that he does not really understand them in terms of their attitudes and priorities.
It is clear as the story progresses that Henry's position among the Convenanters is deterioriating and will certainly end badly -- it only carries on as long as it does because of his unusual military talents. This all ends when the Royalists achieve a crushing victory over the Covenanters at the Battle of Bothwell Bridge. The Covenanter army scatters, but Henry himself is captured by some of the extreme Covenanter faction, who of course blame him as the infidel responsible for their loss of God's favor and prepare to execute him as a traitor; he happens to be saved by Royalists crossing their path, and is sent into exile until the overthrow of the Stewarts.
Scott presents the Covenanters in such a way as to emphasize that fanaticism is their fatal flaw, but he is also very careful to be clear that many of their complaints were entirely justified. They were actively persecuted and oppressed, sometimes by slow social pressures imposed from above and sometimes with outright brutality. In a story with so many oppositions, it's rather interesting that among the characters we most see, there are no outright villains on either side; even the fanatics are mostly just people who are in over their heads and are dealing with the fact that they need to forge some kind of unity despite having a disagreement-riddled religion based mostly on isolated individual Bible study and charismatic preachers -- something that is not entirely their fault, since most of the ways to maintain religious unity are precisely the kinds of institutions and practices that get suppressed in a religious persecution. Individual Covenanters often get censured for their own lack of moderation and deliberate refusal to see the big picture, but it's important to Scott's telling of the story that the Covenanters are not wholly wrong and have many noble qualities, which he sees as having been misdirected as a result of their having been mistreated. The nineteenth-century Scot can entirely relish the excellences of his Covenanter ancestors while fully admitting that they were often in error.
There is a love triangle running through this story. I have not emphasized it, partly because it's constant but not actually important to most of the events in the story, which are primarily driven by broader social and political forces. It does play an important role in showing both Henry and Lord Evandale in a good light, particularly since despite being love rivals they keep saving each other's lives, but while the historical parts of the story are fascinating, I found much of the love triangle story to be aggravating. Edith is a fine character for the most part, but her repeated tendency to keep the triangle going as long as possible, even when it is very obviously best for everyone that she choose Lord Evandale (the only choice she can make that does not end in disaster for her entire Royalist family), and Lord Evandale's patience with her went well beyond what should ever be required of mere mortals. But again, the role it plays in the story is mostly characterization, and for the most part it does not slow down the plot.
Favorite Passage: This is not a particularly quotable book, but there are many fine passages in which Scott attempts to capture the entire spirit of the era in a few strokes, of which this is one.
While the prudent and peaceful endeavoured, like Niel Blane, to make fair weather with both parties, those who had more public (or party) spirit began to take arms on all sides. The royalists in the country were not numerous, but were respectable from their fortune and influence, being chiefly landed proprietors of ancient descent, who, with their brothers, cousins, and dependents to the ninth generation, as well as their domestic servants, formed a sort of militia, capable of defending their own peel-houses against detached bodies of the insurgents, of resisting their demand of supplies, and intercepting those which were sent to the presbyterian camp by others. The news that the Tower of Tillietudlem was to be defended against the insurgents, afforded great courage and support to these feudal volunteers, who considered it as a stronghold to which they might retreat, in case it should become impossible for them to maintain the desultory war they were now about to wage.
On the other hand, the towns, the villages, the farm-houses, the properties of small heritors, sent forth numerous recruits to the presbyterian interest. These men had been the principal sufferers during the oppression of the time. Their minds were fretted, soured, and driven to desperation, by the various exactions and cruelties to which they had been subjected; and, although by no means united among themselves, either concerning the purpose of this formidable insurrection, or the means by which that purpose was to be obtained, most of them considered it as a door opened by Providence to obtain the liberty of conscience of which they had been long deprived, and to shake themselves free of a tyranny, directed both against body and soul. Numbers of these men, therefore, took up arms; and, in the phrase of their time and party, prepared to cast in their lot with the victors of Loudon-hill.
Sir Walter Scott, Old Mortality, Oxford (New York: 2009).