“Closure” is episode nine of season three of Marvel’s Agents of S.H.I.E.L.D.
In the previous episode, “Many Heads, One Tale”, Gideon Malick revealed that the entire purpose of HYDRA was to bring back the most powerful Inhuman born, the being which Simmons encountered, on the alien planet which it had been exiled to, in “4,722 Hours”.
S.H.I.E.L.D.’s infiltration of the ATCU revealed that they are a front for HYDRA, but a largely unknowing one. This episode opens with Rosalind Price and Coulson discussing the matter, including the fact that all the Inhumans taken by the ATCU are missing, when Grant Ward shoots Rosalind in the throat, killing her. This was in revenge for the death of Agent 33 in “S.O.S.: Part 2” (closure is something Ward and Agent 33 talked about between themselves).
Coulson gets as much information as possible from everyone who knew Ward so that he can finally hunt him down and kill him. He’s willing to use any method possible in order to do so, so he puts Mack in charge of S.H.I.E.L.D. whilst he does.
Fitz and Simmons head to the Gloucestershire castle from “Purpose in the Machine” to look into methods of re-opening the portal. HYDRA are already there, and they have the means of opening a portal, as each of HYDRA’s leaders had a piece of the Monolith, and now Malick has them all. What they lack is the means of doing what S.H.I.E.L.D. did -bringing someone back from the other side. They needs Fitz and Simmons’ help to do this, so that they can finally bring back the Inhuman on the other side of the portal. Of course, they aren’t going to simply ask nicely, and HYDRA takes the two prisoner.