SPOILER WARNING: This article contains major spoilers for "Together or Not At All," the latest episode of Agents of S.H.I.E.L.D.
Agents of S.H.I.E.L.D. just reintroduced a plot point from Season 1. In "Together or Not At All," the latest episode, Daisy, Fitz and Simmons stumbled into the Lighthouse's life support system, where they discovered -- or, rather, rediscovered -- gravitonium.
As they explored the life support system hub, Fitz pulled back a thick steel door to discover gravitonium floating through tubes behind it. "Well, I think I know how they're creating the artificial gravity for the station," he said.
"That can't be," Simmons responded in disbelief. "Unless I'm mistaken, and I hope I am, that's --"
"Gravitonium," Fitz finished for her. However, they were unable to unpack the meaning of this discovery, as a Kree warrior rudely interrupted with a hailstorm of bullets.
Gravitonium debuted in the third episode of Season 1, which also introduced scientist Franklin Hall. In the episode, Hall put his life on the line to destroy the element. Even a small amount of gravitonium is enough to toss a launch a tractor-trailer, so Hall decided to destabilize the element after his nemesis Ian Quinn found a large quantity of it. Unfortunately for Hall, his attempt to destroy gravitonium ended with him being sucked inside it. The gravitonium -- with Hall still inside it -- was last seen in Quinn's possession in “Beginning of the End,” the Season 1 finale.
Though fleeting, the scene stands out. On an ensemble show like Agents of S.H.I.E.L.D., every moment matters. As such, the agents' discovery of gravitonium could be like Checkhov’s gun: if it was mentioned, it can and will come back up later. In fact, the synopsis for "The Last Day," next week's episode, may indicate that we'll learn more about this sooner rather than later:
Coulson and the team discover that the most unexpected person from S.H.I.E.L.D.'s past may hold the key to stopping Earth's destruction.
As CBR speculated earlier this week, Hall may have something to do with the Earth's destruction. As such, he could be "the most unexpected person" referred to in "The Last Day's" synopsis. Since May has reunited with Robin Hinton, it seems likely that the Inhuman clairvoyant will answer a few of the show's big questions, including this one.
Airing Fridays at 9 p.m. ET/PT on ABC, Marvel’s Agents of S.H.I.E.L.D. Season 5 stars Clark Gregg, Ming-Na Wen, Chloe Bennet, Henry Simmons, Ian De Caestecker, Natalia Cordova-Buckley, Elizabeth Henstridge, Nick Blood, Jeff Ward, Eve Harlow, Pruitt Taylor Vince and Coy Stewart.