As I mentioned earlier, I'm analyzing a flawed structure as part of an exercise to improve my own writing, specifically using the third "Xindi Arc" season of Star Trek: Enterprise.
Now, let's look at the beginning section of the arc, the first third. This would consist of the second-season finale "The Expanse" as prologue, and the first eight episodes of the third season: "The Xindi", "Anomaly", "Extinction", "Rajiin", "Impulse", "Exile", "The Shipment" and "Twilight".
In three-act parlance, which I'm not a big fan of, this is all Act I. And, as far as I'm concerned, that's part of the flaw here: these episodes mostly serve to put the pieces on the board, and there really aren't that many pieces.
The Prologue set-up of "The Expanse" does its job relatively effectively, but it doesn't quite hold up to scrutiny. Earth is the victim of a Pearl Harbor/9-11 style surprise attack, killing seven million people. The attack comes from a single-occupant probe of unknown origin. Here's where part of the problem comes in: rather than have Starfleet actual figure out its origins, at least in part, answers are handed to Capt. Archer by an exposition fairy from the future. And said answers, as they usual are from poorly conceived exposition fairies, do little more than nudge in the right direction. In other words, the information they gain could have been gained in a less clunky way, and have been just as useful. But "The Expanse" sets the tone, puts a name to the adversary (The Xindi) and a place to find the (The Delphic Expanse). The Expanse, itself, is set up as the equivalent of the "Here Be Dragons" part of the space map: physical laws don't work right there, and it's so dangerous even Vulcans and Klingons steer clear of it.
The true "Act I" of those eight episodes serve to establish a few core elements: the dangers of the Expanse (tied to bizarre spacial anomalies), which may be connected to a mysterious sphere; the stakes for the crew of the Enterprise, personally and globally; the Xindi themselves. The problem is, only three of these episodes really effectively achieve these establishment goals: Anomaly, The Shipment and Twilight. The key points in play in this section are tied to what the crew needs and wants. They want to find the Xindi, but they really don't know where to start looking on once in The Expanse. They need to keep the ship safe from the spacial anomalies they constantly run into in The Expanse. For the latter, there is a running subplot involving Trellium-D, a substance that will protect the ship, but that solution isn't acceptable: Trellium-D is toxic to T'Pol, so they can't use it.
This ties into the only real character arc in this section: what is Capt. Archer willing to sacrifice to save the Earth?* His morality takes a bit of a pounding here, at one point torturing a captive pirate to get the information he needs. He's also willing to pimp out Ens. Sato to a mysterious alien in "Exile", though that's a sacrifice Hoshi volunteers for. He spends much of these episodes at the end of his rope, because he really doesn't see the mission as anything more than a ridiculous long shot. IF they can survive the Expanse, and IF they can even find the Xindi, then they'll still be outnumbered and outgunned. This comes to a head in The Shipment, where they find a Xindi refining facility. The materials made at this facility were used in the weapon of the initial attack, and Tucker, Reed and Hayes are all ready to start blowing things up. Archer, instead, decides to get to know one of the Xindi, and reaches an understanding with a decent man who didn't know what his materials were being used for. Two key things accomplished: The Xindi aren't All Bad, and Archer moral compass veers back towards where Trek's is supposed to be.
The final step in this part is in Twilight, a psuedo-time travel story that nails home the stakes of failure: Earth will be destroyed if the Enterprise doesn't succeed in their mission, which they don't due to Capt. Archer being incapacitated by on of the spacial anomalies. It is a classic Trek "reset button", but one with a point. It doesn't just give us craziness and then undo it.
Even still, the big problem with the arc in this part is it mostly just meanders. It takes eight episodes to do the work of three or four.** Extinction, Rajiin and Exile are largely pointless, serving only to provide us with funny make-up, cheap titillation and a poor Beauty-and-the-Beast homage, in that order.
When it comes down to it, the first section only really does the work of the first two parts of the Twelve Part Arc Structure, when it should be doing the first four. Once the situation is Established in The Expanse/The Xindi/Anomaly, we don't get back on point until The Shipment/Twilight to incite the plot to really start moving. That aimless wandering in between those points is where an audience is lost.
Next up: the mushy middle of the Xindi Arc.
*- The other character arcs set-up here are essentially minor bits of business. Cmdr. Tucker is assigned the personal loss from the initial attack (his sister was among the seven million), but this mostly serves as an excuse to push him romantically towards T'Pol. T'Pol's main conflict is resolved in the prologue: the Vulcan High Command don't want her on the mission, but she resigns from the High Command to stay on board. Of the minor four members of the main cast, only Lt. Reed is given anything: bristling against Maj. Hayes, of the newly added MACO forces.
**- I'll allow that "Impulse" does a fair amount of work of increasing personal stakes, and ties into what the Enterprise can do to keep themselves safe from the anomalies.