Sunday Chess Puzzle, December 5

Lately, I've been trying out 1. Nf3 (as white) and 1... Nc6 (as black) openings. This was inspired by a passage I read about the era of hypermodern school of chess, which includes the idea of allowing the opponent to occupy the center, and punishing them later. This game seemed to justify that style of play.

I was black, and it is black to play. White's position looks solid at first glance, but white's pawn on e4 can actually be taken. Why does white's knight not defend this pawn?