“The Only Light in the Darkness” is episode nineteen of season one of Marvel’s Agents of S.H.I.E.L.D.
At the end of the previous episode, “Providence”, Coulson’s team had found refuge in a secret base that had been established by Nick Fury. Agent Ward had joined them, but they were unaware that he, like Garrett, was a traitor. Although neither Garrett nor Ward seem to have joined HYDRA due to any great belief in the organisation; Garrett seems to have joined purely from opportunism, and Ward simply because Garrett had.
Much of the team set off after one of the people released from the Fridge by HYDRA, leaving Ward, Skye and May at the new secret base. Even though they now that the prisoners from the Fridge have only been released as a distraction. They may just be a distraction, but that doesn’t mean that they aren’t dangerous – there was a reason they were being held there, after all.
Apparently, this episode clears up the matter of the cellist from The Avengers, but I can’t remember anything about that in the film.