After a run of big episodes lately, this felt like a step back towards fundamentals. Most of Clark's plot is exactly that: an attempt to address Clark's ongoing struggle to find his place in the world. It's tied directly into Lana's return to Smallville and the impact that has on Clark's psychological state.
Clark has slowly but surely been seeing the benefits of a more global view of heroism. He's still not comfortable with the kind of above-the-law attitudes that rule Oliver's conscience, but he's been looking past the confines of Smallville and his circle of friends. Even so, he hasn't been forced to consider the downside of the presence of superheroes too often.
Lana has always stood as a personification of everything that holds Clark back. When Lana is around, Clark tends to be more insular in his thinking. Recent events have made him less likely to do that, but his experience with the Metropolis Police forced him to consider how the "capes" affect those bound by the rules. The resultant soul-searching gives Lana an opening.
This would be a bad thing for the growth of the series, and for this season, if it wasn't abundantly clear that this is all designed to make Lana's true final exit more meaningful than the disappointing off-screen farewell of the seventh season. If this wasn't meant as a minor and momentary speed bump on Clark's destined path, it would be a galling slide backwards. As it is, those who were well past caring about any Clark/Lana pairing will be counting the minutes until it's finally over.
For me, the more interesting plot thread involved Lana and Mercy. I never caught on to the fact that Mercy always knew Lex was alive, but in retrospect, I suppose it make sense. That would certainly explain her early interest in Clark. The ocular implants were a lot more interesting, however, because it puts Mercy in a position to take an unexpected direction. It also gives Lex a more active role in current events, which is a nice touch.
The main point to take away from this episode, it seems, is that Clark is struggling with his role in the world, even as others move into position to take the actions he's avoiding. Lana is clearly working for someone standing in opposition to Lex, and her own motivations are in stark contrast to Clark's thoughts in this episode. When Clark learns the truth, it won't just propel him further into Lois' arms; it could be what it takes to drive him towards his destiny once and for all.