Leo Fitz has pined for his best friend Jemma Simmons over the last two and half years and following several separations, the two lab rats are on the cusp of a romantic relationship on "Agents of S.H.I.E.L.D."

"Their relationship will visibly change forever," executive producer Jed Whedon told Entertainment Weekly about Tuesday's episode.

The "S.H.I.E.L.D." creators originally conceived FitzSimmons as best friends who shared more of a brother-sister dynamic. The direction for the characters changed when they witnessed the "undeniable" chemistry between actors Iain De Caestecker and Elizabeth Henstridge.

FitzSimmons still has a few obstacles to overcome, the biggest being Will (Dillon Casey), the lost traveler who Jemma shacked up with for six months when she went missing on a mysterious planet. Fitz promised to help her bring him back to Earth but as the couple grows closer, he'll start to question whether he actually wants to bring Will back.

"Right now, they have no leads," the S.H.I.E.L.D. showrunners told EW. "It will be a long road. The bigger question is whether or not Fitz will continue to want to help as he finds out more about Will and, more importantly, spends more time with the woman he loves."

Fitz and Simmons are ready to express how they really feel about each other and will do so in the next episode.

"This is the episode where they both will express their feelings finally about everything that's gone down," Whedon said.

"Agents of S.H.I.E.L.D." airs on Tuesdays at 9 p.m. on ABC.