Their bond is solidified as they face mortality together. Fitz's refusal to leave the lab despite the risk of infection shows a depth of loyalty that becomes a series-long theme.
The Agents of S.H.I.E.L.D. episode " F.Z.Z.T. " (Season 1, Episode 6) is a high-stakes emotional thriller that transforms the "science duo" Fitz and Simmons from comic relief into the heart of the team. The Mystery of the Static Deaths [S1E6] F.Z.Z.T.
The tension shifts to the "Bus" (the team's plane), which becomes a high-altitude quarantine zone. Simmons estimates she has only before the virus causes her to erupt in a lethal electrostatic burst—one strong enough to take down the entire plane. Their bond is solidified as they face mortality together