Plot summary
The agents end up in 1955 near Area 51, a S.H.I.E.L.D. base currently working on Project Helius, an ion fusion reactor prototype. The agents kidnap high ranking agent Gerald Sharpe and have Coulson impersonate him while Simmons impersonates Peggy Carter so they can find the Chronicom infiltrators. However, they run into Carter's former partner Daniel Sousa, who outs them. Despite putting up resistance and his racism towards part of the team, Sharpe eventually reveals that Helius cannot function without a powerful energy source, leading Deke to deduce that the Chronicoms plan to sacrifice one of their own to activate the weapon and destroy the base.
Daisy arrives undercover and convinces Sousa to release her teammates just as the undercover Chronicoms enact their plan. Daisy and Simmons improve a S.H.I.E.L.D. EMP device in time to disable Helius, along with the entire base, the Chronicoms, and Coulson. Mack and Deke return Sharpe to the desert and pose as aliens to avoid changing history.
Credits
Adapted from Wikipedia contributors, Agents of S.H.I.E.L.D. season 7. HTML formatting and reference markers removed.