I've been pretty tough on Smallville lately – and taken some abuse for it – so let me come right out and say that Thursday's episode, "Descent," was a remarkable return to form. The tone and characterization of this episode reminded me of why I love this show, and made me (for the first time in a while) impatient for more.

"Descent" (7x16) was written by Don Whitehead and Holly Henderson, who are responsible for some of the better recent episodes including "Lara," and not only does it kick into high gear Lex Luthor's long-brewing fall into evil, it makes sense of it. First, Lex's disturbed mental condition is brilliantly manifested by his struggle with his pre-meteor-shower childhood self, Alexander (Connor Stanhope). The device of Lex facing off with Alexander, established earlier in the season in "Fracture," effectively evokes Lex's internal agony, making all the more shocking the revelation that he is finally ready to resolve that dissonance within himself no matter the cost.

Moreover, Lex's final move against his father, Lionel (John Glover), is the act not only of a man increasingly obsessed with a goal (the truth behind Veritas's plan for the Traveler) but of a rejected son determined to resolve his rivalry with Clark for his father's affections – in the only possible way.

In the past it has not always been clear how Lex would eventually become evil, as is necessitated by future continuity, other than through the baleful influence of his unscrupulous father. But now, we realize that Lex's evil derives, tragically, not from Lionel but from Lionel's relationship with Clark.

"Descent" is anchored by a powerful performance from Michael Rosenbaum, and Tom Welling as Clark responds with equal weight, communicating more than ever before that their rivalry has surpassed the childish machinations of previous years and entered a dark and deadly phase. The remaining episodes – of which there are reportedly four left – will be must-see viewing for me and all Smallville fans.