Agents of SHIELD's FitzSimmons are the wonder twins of science, but their friendship will take a turn for the complicated in season 2.

Ward Will Not Become The Punisher On 'Agents Of SHIELD', Will He Escape Custody?

In the wake of the pair's "Beginging of the End" underwater adventure, Fitz (Iain De Caestecker) was left fighting for his life, and now Simmons (Elizabeth Henstridge) will struggle with her own version of survivor's guilt.

"She's changed a lot from what happened at the end of last season, and her relationship with Fitz is obviously strained and fractured due to her guilt that she feels for taking the oxygen and him having no air," Henstridge explained to Buddy TV at the 2014 San Diego Comic-Con. "So their relationship is something that we're going to explore more, and the undertones of guilt and regret."

Avengers To Learn Coulson Is Alive? Clark Gregg Teases Reunion

Although viewers will start of the season knowing that Simmons is burdened by these feelings, it won't be until late in the premiere that Fitz's ultimate fate - and possibly his own opinion on their near-death experience - will be revealed.

Find out more when Agents of SHIELD returns to ABC on Sept. 23 at 9 p.m. ET. Click the video below to see FitzSimmons heartbreaking scene from the season 1 finale.

Tags: Marvel's Agents of S.H.I.E.L.D. television ABC Marvel