"On Guard" is the most impressive episode of White Collar since the pilot. Well, maybe it's actually more impressive. The relationship has been built up between Peter and Neal for two seasons as they've taken down criminals of all sorts. They've come to like each other and they are more like partners than adversaries despite their differences in position, Peter being the fed and Neal being the con.
The end of the second season had a major change, Neal in a room with the art from the German U-boat, but the writers could have taken this any way. In the first season, it looked like Peter was the big bad who had Kate, but it turned out he was not. The opening of the third season reinforces the idea that Neal has completely turned and all ties with Peter will be cut. He and Mozzie load their wares onto a planes and prepare to take off, pursued by the FBI.
Slowly but surely, the writers peel back the layers until we learn what's really going on. And what really happens is more interesting than the implication that Neal is on the loose. Instead, we learn that it was actually Mozzie who stole the art without consulting Neal first. The plane turns out to be part of the case, and Neal sacrificed his chance to escape in order to save Jones. It's not all bright and sunny, though, as there is clearly an underlying deception. Neal knows about the art and is keeping the knowledge from Peter, even swapping out the swatch of the painting Peter got tested.
While everything is ostensibly fine at the end of the episode, their relationship is the closest its been to the verge of collapse. Neal will have to keep the secret, always deceiving Peter, and Peter will always have his eye on Neal. With these circumstances, a lot can happen.