If there was only one word to describe “Harmonic,” the Jan. 18 penultimate episode of “Frequency’s” first (and maybe only?) season — it would be: Dread.

Not necessarily surprising. In fact, any time a series bases its entire plot around tracking down a serial killer, there’s going to be a certain type of unease as the loose ends are tied up before the big finale. But to its credit, “Harmonic” does a good job planting the seeds for its final reveal, as Raimy (Peyton West) and Frank (Riley Smith) believe they may finally catch the Nightingale Killer once and for all.

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The show picks up right where it left off: Raimy drives Megan and her brother Robbie (Beth Lacke and David Lipper) to the hospital, after Robbie was found fatally injured in the woods. Along the way, Raimy tries to get answers out of Robbie — to little avail, until it’s revealed he’s been living under a pseudonym ever since he first ran away from his father decades ago. When Raimy asks if he knows what his father, Joe, did with his mother’s body, Robbie says he can’t remember… But reveals he was apparently there when she was killed, in a particularly disturbing flashback that shows Joe forcing Robbie to help him cleanse her body afterwards.

All of this just seems to continue backing up Raimy and Frank’s belief that Joe’s the killer, and the episode runs on that course for some time. From the discovery of all the killed nurses’ jewelry and name tags in Joe’s private shed, him beating up his own face to get out of Frank’s interrogation, to when he eventually tells Raimy where he buried his wife’s body in exchange for her shoelaces, to kill himself in his jail cell, Joe might as well be walking around wearing a shirt that says “I’m A Serial Killer” in big bold letters all over it — and actor Kenneth Mitchell does a serviceable job at selling the character’s creepiness along the way.

But then, in 1996, Joe turns himself in to the police, and with Raimy’s help, Frank is able to locate the body of Joe’s dead wife, effectively guaranteeing that Joe will spend the rest of his life in prison. Frank is finally able to reunite Robbie and Megan — and Raimy’s mother’s still alive. Frank and Raimy have their happy ending, and it’s time, as Frank advised, to pop open a bottle of champagne and celebrate…

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…If only there weren’t just a few minutes left in the episode.

frequency 1 Frequency tricks us with a happy ending, just before the biggest shock of all
As Raimy celebrates her mom’s return, and — judging by the engagement ring on her finger, a new relationship — we cut away to Robbie again.

At first, we might have assumed that Robbie was about to get murdered in his tiny apartment, and therefore unable to testify against Joe — but with the body of Joe’s wife in the woods near his shed, and the fact that he turned himself in by showing a waitress a box filled with the trinkets of the Nightingale’s victims and pictures from the crime scenes, it would be viable to think that even without his son’s testimony, Joe’s fate would still sealed. So?

“Frequency” has something else entirely in mind for its final shocker: It’s clear immediately that something’s off about Robbie, so when he begins to strip off his shirt, jacket, shoes, and socks mechanically, carefully folding all of his clothes in neat piles on the floor, and makes his way to the closet, his entire demeanor has changed from innocent boy to someone much more strange and disturbing: And the closet, walls filled with images of Raimy’s mom, suggest she’s not so safe after all.

Obviously, this throws basically everything that we’ve learned prior to this almost entirely out the window: Is it Robbie that actually killed his mother and those women? Is this a “Final Destination” situation where her mom is going to die no matter what? Or are we looking at two creeps, working in concert?

It seems unlikely Robbie would be the one responsible for all of those murders, but it might also explain why Frank was wiping fingerprints from the trophies before turning himself in. Was that what he meant when he told Raimy the devil had always been in his son, and how he had failed as a father? (And hey: Why was there an envelope addressed with Frank’s name hidden in Moreno’s car?)

At this point, it seems anything is possible, but it’s safe to say “Frequency” has managed to tee up things nicely for its next episode, which may or may not be a series finale. For those who’ve watched the show evolve — from a simple adaptation of an obscure early-’00s sci-fi film into a much more ambitious and unique story of a girl trying to bring her family back together also — it’s a lot to take in. But even if it doesn’t get another season, “Frequency” is poised to give us a suitable conclusion to its season-long story… Which, if this is the end, is all we could ask for anyway.

“Frequency” airs Wednesdays at 9 p.m. ET/PT on The CW. The season finale airs Jan. 25, and as of their mid-November announcements, The CW has not ordered another season.

Posted by:Alex Welch

Alex is a writer living in Los Angeles, CA who's been digesting too much TV for as long as he can remember. When he couldn't fall asleep on school nights, he used to stay up until 2 a.m. watching "Boy Meets World" on ABC Family. He also believes nothing good happens after 2 a.m. TV words to live by: "Believe in yourselves. Dream. Try. Do good."