ok, so Ryan went in to talk to summer, to make a case for seth. She was resistant, but said that Zach was not her boyfriend, just some guy, Seth finds out that Ryan went there to back him up and is happy, but reads into the fact that she went out of her way to say Zach was not her bf. He thinks that this is a sign that she wants him back. He decides to stand on a cart (like he did last season) and proclaim his love for her. He tries it at the carnival (remember the annual carnival?), but it has disasterous consequences. Summer tries to leave the carnival in Zach's car, but Cohen stops them to try to convince Summer to take him back. Summer then lays it out: "You only want me when you don't have me" She reminds him how he chose Anna, and then when he ditched her at the end of season 1 as examples.
Then there's the Caleb thing. He's in trouble, and Sandy is his reluctant legal counsel. Julie Cooper-Nichol finds out that he;s in trouble with the law, and when she hears that he knew he was in trouble before they got married, she was steaming. She then told Sandy that if Cal was going down, so was Kirsten. The show ends with Cal eating takeout at the Cohen house at dinner, when they are interrupted by the police, coming to arrest Cal. Oh yeah, and the whole Ryan/Marissa thing: He catches her in the act w/ DJ, and then tells her that they should be friends later on in the episode.
that's what happened in the second half...