The Flash finale
The Flash with Iris West, Joe West and Eddie Thawne in The Flash finale promotional pictures The CW

The Flash season 1 finale will be thrilling but it will also break viewers' hearts.

Grant Gustin, who plays Barry Allen/The Flash, teased fans (via Twitter) with the promise that the finale is truly heartbreaking.

Danielle Panabaker echoed similar sentiment and urged fans to brace themselves for a tearful season 1 finale.

The Flash finale will feature Flashpoint Paradox?

Spoilers Ahead

Episode 23, which is the season finale, is titled Fast Enough, where Barry will travel back in time to save his mother.

But changing the past will have major repercussions on the future, and might create an alternative timeline, called the Flashpoint Paradox like in the Comics.

In the comic book event, Flashpoint, Barry Allen travels back in time to save his mother and the results are disastrous. Barry is successful in saving his mother's life from the Reverse Flash, but Nora Allen surviving had wide ranging effects for the universe at large.

The Timeline was so disrupted by the change that the world became almost apocalyptic and terrible. Heroes were villains and lifelong friends were at war.

A new promo for the finale also teases an apocalyptic-type-event.

"I dreamed my whole life about saving her," says Barry in the promo Video.

"This is why you became the Flash" Joe tells Barry, adding, " There is no choice here, Barry. You have to do this, you have to change the past."

But the 1-minute video shows the whole world falling apart, because of the warped timeline.

As Cisco shouts, " he is being erased from existence."

The last scenes shows Barry, at the night of his mother's murder. Nora Ellen says, "Who are you?"

"Its me, mom, Barry," says the flash with tears.

Executive producer Andrew Kreisberg hinted at seeing the ramifications of an alternative timeline where Barry's mother survived, which would likely be the Flashpoint Universe.

Speaking at the Wondercon panel, Kreisberg said: "In the final few episodes, the characters will hear about their future and where they should have been/will go. We will see everyone's reaction to that knowledge and how the characters decide to act on it to alter their own futures."

Panabaker as Killer Frost in The Flash finale

Danielle Panabaker, who plays Caitlin Snow, revealed in a Reddit AMA that she will eventually become Killer Frost on the show, in an alternate timeline.

Panabaker noted: "From the beginning, I knew that Caitlin Snow becomes Killer Frost. And I can't wait to suit up and duke it out with the boys!"

While she didn't reveal exactly when she would receive the power upgrade, Panabaker did say that it was coming soon, and to "watch the season finale" for developments.

"You're going to see her sooner than you think," she teased.

Killer Frost is a stone-cold murderer and supervillain in the DC Comics.

Jesse L. Martin on Barry's decision in Flash finale

Jesse L. Martin told People:" He [Flash] learns a few things in the upcoming episode – particularly going toward the finale – that things may be able to be changed. There's this whole notion that Barry has discovered time travel and if he could go back and change things, would he? That's the whole gist of our finale."

The actor also teases the Flashpoint paradox playing out in the Episode 23.

"Barry has the opportunity to maybe go back and change the way things went down but if he changes anything, that means his relationship with Joe may never exist, with Iris, with all the people he's come to know. But he will have the happy life he thought he'd have with his mom, Nora, and his dad, Henry." Martin added.

Martin explained Barry's decision, and whether or not he will change the past. "So, it's a huge decision for Barry. He goes to Joe to ask for advice and Joe says what Joe would say: "If you have a chance to change your life and to save your parents then you should do so. Don't worry about what that means for us." And the big thing in the finale is whether he does or not."

Here is the official synopsis released by the CW for the finale.

Wells (Tom Cavanagh) presents Barry (Grant Gustin) with a life-changing choice. Dr. Martin Stein (guest star Victor Garber) and Ronnie Raymond (guest star Robbie Amell) return to help the S.T.A.R. Labs team with this final fight. Dermott Downs directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Gabrielle Stanton & Andrew Kreisberg

The Flash's season finale, Fast Enough, airs May 20 at 8 00pm on The CW.