the story: Reed and Hayes butt heads, a mysterious alien appears.
what it's all about: "Harbinger" is very much an episode packed with subplots, connective tissue that moves the season arc along. Its title implies that there is a major development, and indeed there is: the "mysterious alien" in my brief summary is the first time we see a Sphere-Builder, which means the arc has gotten that much closer to reconciling the spheres with the greater Xindi threat.
That's all well and good, but the more important developments in the short term are character-based. T'Pol and Trip reach a hiccup in their relationship, as T'Pol realizes she's developing feelings for Trip. The therapy sessions they've been sharing throughout the season have created a kind of intimacy, but T'Pol was never interested in anything more than that until the events of "Similitude," in which she learns from Trip's short-lived clone that he has feelings for her. Unaware of this, in the meantime Trip has pursued a romantic relationship with a MACO, which...Okay, this sounds like soap opera material.
And the Reed/Hayes material seems equally on the nose. They essentially have developed a professional rivalry, neither quite knowing where their responsibilities end and the other's begins. This actually leads to a fight! But franchise fans won't find this quite so cartoonish when they remember Odo always held a grudge anytime Starfleet suggested one of its officers work alongside him to administer station security in Deep Space Nine. Hayes may never have gotten a chance to expand beyond this moment or his role as a MACO, but this is his biggest spotlight.
Essentially, "Harbinger" is serialized Star Trek in the manner serialized television would come to be known later, even in Discovery. So it's a harbinger of that, too.
franchise- The importance of events in this episode may elude casual viewers.
- series - But they're important for many elements of the season, and even series.
- character - Trip and T'Pol's evolving relationship breaches the top of romance for the first time.
- essential - The first look at a Sphere-Builder is crucial to the outcome of the whole season.
Steven Culp (Hayes)