Eric and Annie reluctantly allow Mary to go out with Robbie, on the condition that they double date with his younger brother Rick (guest-star, Lance Bass, ""N'SYNC"") with Lucy. Meanwhile, Simon and Nigel buy several packs of cigarettes for a class project to prove how easy it is for children to buy tobacco. Also, Ruthie works out her own secretive science project and John invites some female neighbors over for dinner to take Matt's mind off of Shana.

It's Valentine's Day and the Camden's are celebrating the day of love with a special party in honor of the twins first birthday. Eric and Annie fret over how to make the babies birthday a memorable one, while the rest of the family learn different ways of exchanging love on Cupid's Day. After finally gaining her parent's trust, Mary is allowed to spend a romantic dinner with Robbie, only to learn he has ulterior motives. Meanwhile, Simon and Deena confess to their parent's that they've given each other love bites. Also, Matt welcomes his ex-girlfriend Heather back into his life, threatening his relationship with Shana.

Eric and Annie must deal with the emotional fallout as they learn about their kids tumultuous love lives. Mary upsets her parents by announcing that not only has she decided to skip college in the fall, but she is getting back together with Robbie. Matt is forced to choose between his girlfriend Shana (who has moved back to Glenoak for the summer) or his ex-flame Heather, who confesses that she still loves him. Lucy dates Robbie's cute brother Ronald, disappointing Andrew Nayloss who still harbors feelings for her. Also, Simon gets dumped by Deena and Ruthie takes advantage of her new boyfriend.

In the fourth season finale, Simon finds out the real reason why Deena wants to break up with him. Also Ruthie does her best to get back together with her old boyfriend Burt, while Lucy also tries to go after an old flame: Andrew Nayloss. Mary decides to take things slow with her relationship with Robbie and her life. Matt and Shana break up their relationship, but reconciles with Heather. At the end of the show, it shows Matt and Heather may have gotten married- but didn't.

There is cause for alarm when the Camden's disapprove of Simon's new group of friends, one of whom insults Annie and Lucy with his chauvinistic remarks. Norton, the kid that Simon befriends is a fan of violent rap whose negative attitude towards women seems to be influenced by misogynist lyrics. At the same time, Eric and Annie confront Simon about his newfound interest in rap music and somewhat misogynist attitude towards women. Heather gets back in touch with Matt, but Matt is unsure of her intentions. He then finds out that she is dating someone else AGAIN!

Mary's feelings for Robbie resurface when she inadvertently finds out he has a secret date for Valentine's Day, and the whole family schemes to uncover the mystery woman. Meanwhile, with the exception of Lucy who has a date with Jeremy, the other Camden kids feel deflated when their Valentine's plans fall through and they end up stuck at home celebrating Sam and David's second birthday. Lucy and Jeremy's first date does not go as expected as they both keep on running into Lucy's ex-boyfriends. Although all's well that ends well by the end of the episode for everyone.

The striking resemblance of Robbie's new friend, Marie, to Mary raises eyebrows among discerning Camdens. Annie's invitation for her to stay for dinner makes things weirder for them. Back in Buffalo, unbeknownst to Robbie, Mary accidently bumps into her old boyfriend Wilson (guest-star Andrew Keegan) and his son Billy at the train station and makes plans to meet him for the evening. Mary is unsure on whether she should tell Robbie, but confides in her family about this. They all say that she should tell Robbie about this, but decides to ignore their advice. Also, Matt meets Robbie's ex-girlfriend, Cheryl, at the restaurant where she works. He starts to have feelings for Cheryl and tries his best to try to get her to go on a date with him. His efforts work by the end, but is having a hard time telling Robbie about his interest in his ex-girlfriend.

Annie discovers that the particularly flirtatious mother of one of Lucy's friends, Serena is seeing Eric for therapy sessions. Annie strongly requests that he refers her to someone else. Annie then has to deal with the guilt over her jealousy-induced interference between the two. Also, Simon is mortified when word gets around school over the fact that he's a virgin. Annie had shared the details of Simon's sexual inexperience with his ex-girlfriend's (Sasha) mother, Rita. Meanwhile, Matt struggles about telling Robbie he's interested with his ex-girlfriend, Cheryl. Lastly, Robbie is also hiding a secret from Matt that involves his ex-girlfriend Heather.

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.

Simon decides to stand up for bullied student at school only to have the boy tell him that he is going to shoot his tormentors. Robbie decides to give Ruthie a playful new nickname, but at the same time Ruthie comes up with a less-pleasant one for Lucy after breaking off her engagement to Jeremy in New York. Eric gets a visit from an old friend Merle with some family issues of her own. Mary gets serious with pursuing the career as a firefighter, but things don't turn out the way she had planned.

As Annie begins teaching, she inadvertently gives a student some bad advice. At the same time, Eric and Annie painfully discover how much their constant arguing is affecting the family. Lucy and Robbie finally face up to their feelings after she intentionally sabotages one of his dates out of jealousy. Cheryl goes to Eric for some guidance in her relationship with Matt, but she misinterprets the reverend's input in the matter. Simon is attracted to a girl who is dating one of his friends. Mary is left to mull over Wilson's marriage proposal and it's even more confusing when she has a romantic encounter with a fellow firefighter trainee who shows an interest in her.

Lucy and Robbie argue over who should tell Mary about their emerging relationship. Meanwhile, Matt worries that a young woman in his study group might be a victim of physical abuse at the hands of her belligerent boyfriend. Eric's sister Julie shows up at the Camden house with her toddler Erica and annoys Annie as she tries to prepare a lesson for her class. Simon meets up with Sasha and they both try to catch up on their personal lives.

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.

Eric begrudgingly agrees to see a therapist in order to appease Annie who is at her wits' end with him. Meanwhile, Simon is ecstatic when Cecilia's father agrees to let him see Cecilia, but feels hurt and confused when she starts avoiding him. Roxanne becomes suspicious when a pretty girl seeks council from Chandler. Lastly, Ruthie is shocked to learn that her new upbeat friend Katelyn suffers from cystic fibrosis.