Obviously this episode ("Tabula Rasa") is about fresh starts. It's the first ep to use extensive flashbacks from before the crash focusing on just one person (Kate); hence bringing up the baggage of her past. Meanwhile, on the island, the seriously injured Marshall represents someone who has a present day tie to Kate's past.
1. Kate doesn't tell Jack about her past.
2. Jack confronts Kate with the fact he knows what she did. // Kate is caught (in backstory).
3. The Marshall wants to talk to Kate, thinks Jack has been compromised.
4. The Marshall leaves the A-plot of Lost.
Cool stuff: the way all the Kate-Jack UST is vapourised by her early lack of honesty. The focus of the episode is very much on their relationshipand about playing what seems to be a low-key but major twist in it. The way there's no clear leadership amongst the survivors.
Later eps of Lost feel like a TV show. This feels like just watching interesting stuff happening. It's not doling out too many plots twists or starting to over-dramatise more than a few characters by involving them in soap-opera issues. The show's just letting people be people.