Exasperation reaches an all-time high in the Camden household, as Eric and Annie helplessly watch their oldest daughter Mary and her worrisome friends continue to head down the path of destruction and into the hands of the law. When Frankie (guest-star Chyler Leigh) expresses interest in signing up for college classes Mary starts checking out Lucy's early application packets. Meanwhile, Simon has a hot date and Lucy gets a weird proposition from a classmate. Lastly, Ruthie discovers manipulative fun of keeping a diary--- much to the embarrassment of Eric and Annie when she sees them having SEX!

Mary's future may be resolved, but the rumor mill surrounding her situation runs rampant. While ugly rumors spread about Mary's sudden departure, Simon is persuaded to use his sister's bad reputation to boost his 'cool' quotient at school and gets into some trouble of his own with identical twin bad girls, Carol and Sue Murphy (Mary Kate and Ashley Olsen, Full House). Congregational gossip irks Eric. Lucy wrestles with high school hearsay. Ruthie spreads phony tales to draw attention away from Mary, and Matt's infatuated with a beautiful stranger. Also, Annie goes out with an old boyfriend.

Despite the disapproval from the rest of the family and his own hesitations, Eric takes in Mary's ex-boyfriend Robbie Palmer (Adam LaVorgna) to live with the Camden's for the night after learning that he's homeless. Matt also shows up unexpectedly after being kicked out of his apartment by John, who needs quality time with his girlfriend. Meanwhile, Mary calls home again and hears Robbie's voice on the other end and assumes he's dating Lucy now. Also, John proposes to his girlfriend Priscilla in front of Matt.

As Annie plans a mysterious day of cleaning as Eric discovers the shocking news that Mary ran away from ""The Colonel""'s to come back home because of a devious scheme she had with Robbie and freaks out. Eric is convinced that more trouble is brewing and goes to insane lengths to find answers. In reality, Mary is heading home for a huge surprise birthday party for Eric, attended by all the people whose lives he's changed for the better in the past few years. Chores at home are interrupted, however, when Ruthie discovers Frankie and Johnny's baby, Mercy at the Camden front doorstep. Meanwhile, Lucy stays with her friend Mike's (guest-star Jeremy Lelliot) near catatonic mother for the day, and drops a proverbial bomb on Lucy. Also, Matt and Simon try to figure out why they delivered Mrs. Bink (guest-star Eileen Brennan) to the hospital.

After their lackluster attempt at kissing, Mike (guest-star Jeremy Lelliot) and Lucy agree to be friends and to fix each other up on dates--- but the plan bothers Lucy more than she anticipated. Meanwhile, Matt's in a huff over John and Robbie's newfound friendship and feels slighted by the family. Eric also continues to feel alienated from his family when they keep going to Annie for advice. Also, Annie feels slighted when Ruthie plays an un-funny joke when the twins recognize her as their ""mama"".

Matt begins to go crazy over a psychological evaluation that he is required to take for his medical-school application. Matt also notices the strange demeanor of his psychologist, which leads to a surprising twist. Meanwhile, after Ruthie gets into trouble at school for missing homeroom classes daily, she tricks Robbie into coming to talk to her principal. Simon decides he no longer wants to be together with Sasha since they have nothing in common and that she is not that intelligent, but the plan backfires on Simon. Also, Mike wants to try to be in a relationship with Lucy again, but unfortunately she turns him down since they are both on the rebound from Jeremy and Elaine. Lastly, Eric must convince a terminally ill patient to embrace life no matter what the doctors tell him.

Mary returns home back to Glenoak for the summer to spend some time with her family, but fears about being together again with Robbie. Eric and Annie are recipients of what may or may not be advances from unmarried acquaintances. Simon and Deena see that they still have feelings for each other, even though she has a boyfriend. Matt agonizes over telling his family about his new relationship with Cheryl, but soon decides to tell them all. Lastly, Lucy surprises herself and others when making a proposal to Jeremy and the possibility of going to college in New York with Jeremy. Although plans change when Mary, feeling rejected by Robbie, makes a shocking declaration.

Mary tries to right a wrong by explaining to Lucy that she never had adult relations with her sister's fiance. The only problem left is trying to convince Lucy. Wilson unexpectedly drops by the Camden home from New York to see if he and Mary are really over by finding out her answer. James talks to Eric about him attempting to have an affair with Annie as Serena comes clean with Annie about her trying to have an affair with Eric. Robbie and Wilson set aside their differences so that they can talk about Mary. Matt learns a few things when it comes to Cheryl's honesty when she doesn't tell her friends that she's dating him.

St. Valentine's Day delivers a few surprises for the Camden family. A grounded Simon is asked out by a senior. Annie's attempt to surprise Eric with romantic Valentine's plans backfires and results in a big fight. Mary discovers Robbie's plans with Joy. Lucy and Mary learn that Jeremy and Wilson both have dates for Valentine's. Matt seeks his fathers help as he wants to settle down, which leads him to seek out old girlfriends as to why they're no longer together. Ruthie is hesitant to go to a party because her friend tricked her into asking out a boy she's already going out with.