Series 1, Episode 21
As Boone is laid to rest some of the group are not in a particularly forgiving mood with Locke, as the finger of blame seems to be pointing towards him. Shannon is convinced he is behind the 'murder' and as such she asks Sayid to "do something about it". The back story shows Sayid being coerced by government officials to infiltrate his brother - who is a wanted terrorist - in exchange for information leading to the whereabouts of Sayid's lost woman.
The show has stepped into a near gear with this episode. It has got a bit 'internal' in you know what I mean!