Clark Gregg reprises his role as Coulson from the film series, and is joined by principal cast members Ming-Na Wen, Brett Dalton, Chloe Bennet, Iain De Caestecker, Elizabeth Henstridge, and Nick Blood.

The episode received a positive critical response, with the performance of guest star Kyle MacLachlan praised, and the introduction of the Inhumans, along with the character reveals, highlighted as groundbreaking and exciting, though also potentially confusing or insignificant to non-comic book fans.

A contact in San Juan reveals to agents Bobbi Morse and Lance Hunter that Hydra has already arrived, setting up residence above the ancient temple said to be needed for an extinction-type of event.

Realizing who Cal and Skye are, and that Ward isn't loyal to Hydra, Whitehall has the three incapacitated and guarded, but is prevented from taking further action when S.H.I.E.L.D.

Director Phil Coulson attacks the base, along with May, Morse, and Hunter, in hopes of disabling the drill before Hydra can reach the temple with the Obelisk.

Agents Antoine Triplett, Jemma Simmons, and Leo Fitz enter the city and place Howling Commando explosives around the temple, ready to destroy it for good.

Skye, wanting to make things right, goes looking for the Obelisk, and realizes that Raina, who Cal also believes to be descended from the gifted people, has taken it down to the city.

Skye bursts out of her own cocoon, her new abilities shaking the entire island, but she watches in horror as Triplett's petrified body crumbles in front of her.

In November 2014, Marvel announced that the tenth episode of the season would be titled "What They Become", to be written by executive producer Jeffrey Bell, with Michael Zinberg directing.

[1] Talking about the revealing of Calvin Zabo and Daisy Johnson in the episode, executive producer Maurissa Tancharoen said, "As we always do, we pulled from what exists in the Marvel Universe and put our own spin on it.

"[2] Elaborating on that and the idea of introducing the Inhumans to the MCU in the episode, Tancharoen said, "It's been a property in the Marvel Universe that we've been interested in since the beginning.

Also, speaking on the death of Agent Triplett in the episode, something the executive producers do not "take lightly", Whedon explained, "We need to establish that there are stakes in our world, and there can be real consequences.

"[2] In November 2014, Marvel revealed that main cast members Clark Gregg, Ming-Na Wen, Brett Dalton, Chloe Bennet, Iain De Caestecker, Elizabeth Henstridge, and Nick Blood would star as Phil Coulson, Melinda May, Grant Ward, Skye, Leo Fitz, Jemma Simmons, and Lance Hunter, respectively.

[1] It was also revealed that the guest cast for the episode would include B. J. Britt as Antoine Triplett, Adrianne Palicki as Bobbi Morse, Henry Simmons as Alphonso "Mack" Mackenzie, Ruth Negga as Raina, Patton Oswalt as Billy Koenig and Sam Koenig, Reed Diamond as Daniel Whitehall, Kyle MacLachlan as The Doctor, Jamie Harris as man, and Hunter Seagroves as Hydra agent.

Sepinwall particularly praised the Skye and Cal reunion scene and the performances of Bennet and MacLachlan, while noting that Triplett's "sacrifice probably would have meant more if he had been given more to do this season".

"[16] James Hunt of Den of Geek praised the introduction of the Inhumans, and the way that the episode tied up multiple storylines, whilst setting up new and exciting ones for the rest of the season.

[17] Kevin Fitzpatrick of ScreenCrush felt that, from a casual viewer's perspective rather than a comics fan's, the Daisy and Cal and Inhuman reveals were most likely insignificant, and perhaps made the episode a bit of a let-down in terms of a mid-season finale.

hasn't seen an episode so simultaneously satisfying to longtime comic-book fans and frustrating to its general audience as it does in "What They Become"", noting that the Daisy and Cal and Inhuman reveals would mean nothing to average viewers, as opposed to "Marvel maniacs" who "should be elated".

McCabe did have praise for MacLachan's performance, calling his reunion with Skye "the most entertaining" scene of the season, and saying "He generates humor, rage, joy, spite, menace, sympathy, and insanity all in the span of a few minutes.