It's sure to have a long-lasting effect on one main character.
The second season has been all about Elliot Stabler going undercover deep in the criminal underworld, eventually ending up as a high-ranking member of the Brotherhood, led by Frank Donnelly , an old friend from 20 years ago. Over the season, Donnelly has seen Stabler as his closest friend, making him leader of the Brotherhood and naming his son after him.
That all came crashing down in the finale, 'Friend or Foe', where Donnelly learned that Stabler was an undercover agent. After alerting the rest of the Brotherhood and trying to assassinate Stabler in an ambush, Donnelly escaped. Stabler went to Donnelly's wife Bridget and planned to use her as bait to lure Donnelly out, and she retorted that, even though Stabler works for the authorities, he's no better than the criminals he's trying to take down.