One of the best aspects of Marvel's Agents of S.H.I.E.L.D. is that the series is structured much like the comic books it's based on. Each season follows an "event" storyline that builds up into the ones that follow. The result is a series that brings the fans back season after season to see what's going to happen next, which popular characters from the comics will make an appearance, and how closely the series relates to the books.
Marvel's Agents of S.H.I.E.L.D.'s story arcs are called pods, and most seasons have one large one while others have two to three. Pods have been described as "7-10 episodes [that feature] little 'enclosed plotlines' that formed a larger whole." This structure will continue as the show enters its sixth and seventh seasons. Each pod has its fans, but some are considered significantly better than others.
What Happens: "Agents of HYDRA" is the third and final pod from the fourth season. With the main characters stuck in Aida's framework, life looks rather different. S.H.I.E.L.D. has fallen while HYDRA has taken its place, and the woman in charge is none other than Aida, otherwise known as Lady Hydra within the Framework. At her side is Fitz, whose father is still around to lead him down a path of darkness as Aida's righthand man.
Coolest/Most Bad-Ass Moment: When the team finally escapes in a massive fight at the very end.
What It Sets Up: The inevitable confrontation with Aida in the real world. However, things aren't as simple as they used to be thanks to the Darkhold...
What Happens: In the final two episodes of the fourth season, the team must find and deal with Aida, but there's a catch. She has found a way to make herself an Inhuman body in the real world, filled with the various powers she took from others within the Framework. As a result, catching and stopping her isn't as easy as it was previously. This arc also sees the end of the Framework.
Coolest/Most Bad-Ass Moment: When the Framework comes to an end for Radcliffe and he utters the line, "This is the way the world ends. Not with a bang, but a..."
What It Sets Up: The end of the fourth season establishes the events that build into "Lost in Space."
What Happens: After being taken from a diner after finally getting out of the Framework, all but Fitz find themselves zapped to what appears to be a space station via a strange white monolith. When they awaken, they face a dangerous alien threat called the Vrellnexian. Eventually, it's revealed that they are on a habitat run by the Kree, the same race responsible for the creation of the Inhumans. It turns out that they aren't on a space station at all, but rather, they are in the future sometime after the Earth was wiped out, leaving the survivors to languish on the station run by the Kree.
Coolest/Most Bad-Ass Moment: The opening scene when the crew finds themselves in space and has to find a way to survive without knowing what's going on.
What It Sets Up: A return to the right time, the saving of the world, and the beginning of what will become the "Graviton" event.
What Happens: HYDRA has been thwarted, but S.H.I.E.L.D. is in ruins. As the second season of the series kicks off, Coulson is appointed as director with the seemingly impossible task of rebuilding the once-proud organization. Most of S.H.I.E.L.D.'s roster has perished or is working secretly for HYDRA, which makes returning S.H.I.E.L.D. to its former glory almost impossible. As a result, Coulson handles things differently and keeps things small - but will the team be able to operate with so many threats looming on the horizon?
Coolest/Most Bad-Ass Moment: Just as she is looking like the greatest villain in HYDRA, Bobbi Morse flips and shows she was working for S.H.I.E.L.D. all along!
What It Sets Up: The rebuilding of S.H.I.E.L.D. establishes the events that lead into "Inhumans."