We flashback to three years ago on this episode of Terriers, learning a lot more about why Hank and Gretchen got divorced and how Hank and Britt first met.
As to the former issue: Hank, a full-fledged alcoholic, got caught up in the case of a serial rapist while still working as a detective with Mark. The son of a rich California family was considered a suspect and when Hank asked Gretchen if she knew him while the two were at USC, she reacted poorly. She made it clear that something went down between the pair, but she didn't want to iscuss it.
This caused Hank to believe the guy was the rapist, a fact he went crazy trying to prove. He badgered the suspect. He got drunk. He was questioned by Mark for possibly running this guy off the road and planting evidence in his car, a charge Hank denied, but in an angry way.
The rich dude ended up getting Hank kicked off the force, and it's easy to see how this led to more drinking and more problems with Gretchen.
Hank also met Britt during this case, as he was caught breaking into a car near where one of the rapes took place. Hank got a good vibe from him and gave the criminal his card.
How did all this relate to the present day? We watched as Britt took Winston and left Katie. He was drunk and heartbroken himself, finding a photo of Katie and her classmate from the night she cheated on him. Britt assumed that was the guy, tracked him down and beat him up. But Hank had to break the news to his arrested friend: Katie actually cheated on Britt with her professor, something Hank has known all along. Britt wasn't happy to learn this.
Hank also received help in tracking down the actual rapist from his new reporter friend. The culprit ended up being Mark's new partner. Didn't see that one coming!