Buckle up "Arrow" fans—this Wednesday's installment of the series may be "the biggest episode" ever, says executive producer Marc Guggenheim.

During last week's episode, "Time of Death," Oliver was hit with a bombshell when he found out that Slade Wilson is still alive and is a threat to Starling City. Not only will episode 15 pick up where this cliff hanger left off, but it will also feature a flashback revealing how the former friends-turned-foes got their start.

"This episode, apart from just being massive, features two iconic moments in the life of our series," teased Guggenheim in a preview video about the upcoming episode, titled "The Promise."

"It's the first time that Oliver puts on the green hood and it's the first time that we see Slade Wilson put on the death stroke. And it's the beginning of the relationship that will define the end of season 2," he continued.

In an interview with TVLine, "Arrow" star Stephen Amell also revealed that the next episode will take "the show to another level."

"Our 15th episode, titled 'The Promise,' is this season's version of 'The Odyssey,' in that present day in Starling City takes place over the course of 40, 45 minutes, but a lot of time passes on the island," said Amell.

"It's the episode that I am most proud of. A lot of people have written or said that there are a couple of episodes in the second half of our first season - 'The Odyssey,' 'Dead to Rights' - that took the show and elevated it to the level that we've gotten to this year. I believe that our 15th episode - and possibly our 13th episode that just aired - are the episodes that take the show to another level," the actor added.

In a 30-second promo, Slade tells Oliver that he made a promise back on the island, and now he's here in Oliver's home to bring to fruition. Sara mobilizes the forces, bringing Roy to the fight so that they can pit two super-soldiers against one another, while Oliver and Slade mutter threats to each other while trying to keep up appearances for Thea and Moira.

Promotional photos from the epiosode show the modern-day Slade Wilson introducing himself to the entire Queen family—even Roy Harper—while Oliver later steps up his training of the youth to battle his former foe, reports Screen Crush.

In another picture, the past Slade doesn't look too happy to discover the truth about Oliver's role in Shado's death.

According to ComicBook, the CW released the official synopsis for "The Promise," which reads:

"Oliver (Stephen Amell) is shocked when he learns that Slade (Manu Bennett) has come to Starling City. Back on the island, as Oliver, Slade and Sara (Caity Lotz) prepare to take over the freighter from Ivo (guest star Dylan Neal), Sara pulls Oliver aside and tells him he needs to kill Ivo to prevent him from telling Slade what happened with Shado. Oliver feels immense guilt over the situation but realizes Slade's growing rage from the Mirakuru makes him unpredictable and he could turn on his friends if he thinks Oliver is to blame. Oliver agrees to kill Ivo and the battle begins."

S2E15 "The Promise" airs Wednesday at 8 p.m. EST on CW. Watch sneak peek clips below.