Ryan thinks about not inviting Marissa to his 18th birthday after their falling out but Sandy tries to get him to invite a relative. After months of avoiding telling Summer and Marissa the truth Neil and Julie tell Summer and Marissa the truth.

Julie and Dr. Roberts announce their engagement and the Cooper ladies move in with Summer and her dad. Meanwhile, Marissa drifts away from the group and Summer when she seeks solace in the bad company of Volchok and his crew. Ryan tries to move on with Sadie, but Seth's concerns for Marissa get in the way. At the Newport Group, Sandy's woes with Matt continue and Kirsten surprises Seth with a night out.

Ryan decides to go and invite his mother to his graduation party. Taylor and Summer work together to try and find out what Seth is hiding, but Summer ends up getting hurt. Marissa realises she is in bad company after she finds out how low Volchok's friends can go. Sandy is able to fix his work problems with Matt but while doing so he hurts Kirsten.