The time travel in season 5 of Marvel's Agents of S.H.I.E.L.D. seemed relatively straightforward: the Agents were abducted and sent 78 years into the future via a Kree Monolith, where they were prophesied to save the world. (Fitz later joined them by taking 'the long way' - cryogenically freezing himself.)
This week's episode 'The Last Day' complicated matters by introducing flashbacks which indicate that our heroes did already travel back in time to prevent Earth's destruction - and couldn't. It even posited that S.H.I.E.L.D. may have attempted and failed to save Earth numerous times in different timelines.
Here's what we do know for sure: Phil Coulson, Melinda May, Daisy Johnson, Mac MacKenzie, Yo-Yo Rodriguez, and Jemma Simmons were all kidnapped from the diner at the end of season 4 by Enoch, who was working with the True Believers. The True Believers are the people in the future who rallied around Robin Hinton, an Inhuman seer whose visions created the prophecy about S.H.I.E.L.D. saving the world that was destroyed, which apparently happens in 2018. The diner was the last time the Agents of S.H.I.E.L.D. were publicly seen together, implying that one or more of the Agents either don't make it back from the future or don't survive Earth's cataclysm. (The two main suspects are Coulson and Mac.)
The device the True Believers used to transport the Agents was an isochronus cyclotron with resonant superconducting coils. Housed in the Zephyr, which survived into the future, it was built by Deke's father Owen from old schematics by Fitz. The cyclotron controls the Kree Monolith, which opened the portal that brought the Agents to the future. Only a small piece of the Monolith remains, and it's insufficient to return Coulson and friends to the past, but there is likely more out there. And we know there must be, because we know some of the S.H.I.E.L.D. agents do make it back to 2018 - just in time to watch the world end and create a new timeline (possibly more than one).
By 2022, the Kree have arrived on Earth and 'rescued' humanity by subjugating the remaining people inside the Lighthouse, including May, Jemma, Fitz, Yo-Yo, and young Robin. May has become Robin's 'mother' (her real mother Polly didn't survive), while Fitz is frustrated by their inability to alter time, which he considers 'fixed'. Robin has also told Fitz of her visions that Jemma dies and their great love story ends in tragedy, which only agitates him further.
Why did the True Believers bring the Agents of S.H.I.E.L.D. to the future in the first place? To kill Daisy Johnson. All the available evidence points to the world being quaked apart by Quake - despite Daisy's protestations that she isn't powerful enough to tear a planet to pieces. The True Believers brought S.H.I.E.L.D. to the future in part to eliminate Daisy - the hope is that by ripping the Agents from the timeline before 2018, they could take Quake out before she can cause the destruction of Earth. The True Believers failed to kill Daisy, however, but instead Voss, their leader, stabbed the elderly Robin in the abdomen, which led to hear death and robbed everyone of her visions.
However, if Quake really does destroy the world, it seems the universe provided a counter balance: Flint, the new Inhuman who can move rocks with his mind. Could Flint have the power to restore the Earth back to its original form? Or is Flint going to be revealed as the true Destroyer of Worlds? We are shown footage of Daisy in 2018 telling everyone on the Zephyr to take off while she remains behind. Soon after, the Earth is destroyed. But what if Daisy was really on her way to fight Flint? What if Flint is the one who destroys the Earth - or he and Daisy together accidentally do it in their battle?
As Agents of S.H.I.E.L.D. - the best reviewed Marvel series - rockets towards its mid-season, no doubt the answers will shock fans. Given how unpredictable season 5 has been, those answers will likely bring forth even more questions.
Marvel's Agents of SHIELD airs Fridays @ 9pm on ABC.