Last week’s installment of “The Flash” ended with the arrival a very unexpected visitor — the Reverse Flash (Matt Letscher), aka Eobard Thawne, the time-travelling villain who killed Barry Allen’s (Grant Gustin) mother, leaving Barry’s father to do time for the crime.

Although the Reverse Flash was erased from existence in last season’s finale, following a heroic sacrifice from his ancestor Eddie Thawne (Rick Cosnett), “Flash” executive producer Andrew Kreisberg offered Variety a compelling explanation for his reappearance, which will be explored in the January 26 episode, aptly titled “The Reverse Flash Returns.”

“One of the more interesting aspects of time travel is that time doesn’t unfold linearly. This is the first time for our characters that they’re meeting somebody out of continuity — because this is the Eobard Thawne before he killed Harrison Wells; before he got stuck in the past; before all the terrible things he did to our team last year; and even before he killed Barry’s mother,” Kreisberg explained. “So when they meet him, he’s not the man they’ve come to despise and hate. But they still have that sense of betrayal and that palpable anger at all the terrible things he’s done to them — he just hasn’t done them yet.”

According to Kreisberg, that presents Team Flash with an interesting moral quandary: “They’re faced with, ‘well, if we stop him now, will all of that bad stuff not happen to us?’ And as so many time-travelers and people who’ve been confronted with time paradoxes before them have discovered, time does not want to change.”

While this year’s villain, Zoom, is possibly even more terrifying than Reverse Flash, Kreisberg said the writers couldn’t pass up the opportunity to revisit Eobard Thawne. “The Reverse Flash is our Flash’s Joker … he’s responsible for Barry’s core wound, he killed his mom. These could very easily be the dog days of winter where we tread water for a few episodes until we get the main story kicking again, and we didn’t want to do that,” he admitted. “We wanted to have episodes that have meaning and have weight, and really meant something to the characters as they go on the journey that they go on for this year. So to be able to have the Reverse Flash back and really have him have an effect on Barry felt like a great opportunity.”

The hour was penned by executive producers Aaron and Todd Helbing, and Kreisberg said that “The Reverse Flash Returns” is one of the show’s most powerful episodes yet. “This one just feels really special; it has the best villain, it has Speedster versus Speedster, and it also has all the heart and emotion that I think all our best episodes have,” he teased.

That emotional throughline is thanks to Iris West (Candice Patton) and her developing relationship with her newly-discovered brother, Wally (Keiynan Lonsdale), as well as her attempt to reconnect with her estranged, terminally ill mother Francine (Vanessa Williams).

“Francine’s health has taken a severe turn for the worst, and this is really going to be Iris’ last chance to mend fences with her mother,” Kreisberg previewed. “But what’s really great about the episode is Wally is keeping his distance because he can’t face his mother’s loss, and it really is because of the love of his sister and her own ability to forgive Francine that Iris is able to bring the two of them together. So it’s a really strong Iris episode also, which are always our favorite episodes.”

“The Flash” airs Tuesdays at 8 p.m. on The CW.