When it comes to the Marvel Cinematic Universe, Agents of S.H.I.E.L.D. and its sister series on Netfix have always been outliers. While the television dramas tend to react to MCU events, they have little to no influence on what happens in the movies themselves. Over the years, S.H.I.E.L.D. experienced some growing pains as it tried to strike a balance between its own stories and the films' game-changing turns. With Avengers: Infinity War, the show accomplished its best crossover yet by building the gravity of the blockbuster's situation into the season and giving the team a villain powerful enough to rival Thanos himself.
For the sake of comparison, let's take a look at how the show handled major MCU events in the past. The first -- and very brief -- crossover took place in "The Well," the eighth episode of Season 1, which opened with an unglamorous look at the life of a S.H.I.E.L.D. agent, as the team was assigned to clean up Greenwich after the events of Thor: The Dark World. However, this minor "crossover" of sorts lasted only a few moments before seguing into the discovery of an Asgardian artifact from Norway called the Berserker Staff. While the episode tied into The Dark World thematically, it breezed over the actual events of the film pretty quickly.
Those disappointed by the dismissal of The Dark World found some solace in Captain America: The Winter Soldier, which fundamentally altered the show moving forward. Like The Winter Soldier, Season 1's 17th episode, "Turn, Turn, Turn," revealed Hydra had infiltrated S.H.I.E.L.D. As a result, the covert organization was dismantled and the team's roster, shaken up after they discovered Grant Ward to be a Hydra sleeper agent. The show felt the effects of The Winter Soldier's fallout for years, up through the recently concluded Season 5.
However, The Winter Soldier may have hurt S.H.I.E.L.D. more than it helped. Until such time as the Hydra twist was revealed, the show was stuck twiddling its thumbs, more or less. Prior to "Turn, Turn, Turn," the series bided its time with the mystery of Coulson's resurrection, team building and a rather straightforward procedural narrative, where the team searched the globe for 0-8-4s, or “object[s] of unknown origin." That simply wasn't enough to hook the audience it brought in with its pilot episode; the ratings dipped by more than half between the series premiere and the Season 1 finale. While S.H.I.E.L.D. fans hail "Turn, Turn, Turn" as the narrative turning point for the series, it was just a little too late, and the show suffered for it, through no fault of its own. So, while that is certainly one of the best MCU crossovers the show has pulled off, it wasn't wholly effective.
Agents of S.H.I.E.L.D. played a more active role in 2015's Avengers: Age of Ultron, which featured a scene where Nick Fury shows up in a Helicarrier and saves the day by evacuating Sokovian refugees from the doomed city. As explained in the Season 2 episode "Scars," Fury and Coulson secretly worked together to rebuild a damaged Helicarrier spared from the carnage of The Winter Soldier and called it the Theta Protocol. Overseen by Sam Koenig, the Theta Protocol existed as a fail-safe to be used in the event of a world-ending scenario. When Ultron attacked Sokovia, Coulson and Fury agreed it was time to enact the Theta Protocol.