Mack and the gang are having breakfast when Steve walks into the dining hall. Raffi, Todd and Sheila think it's high time Mack gave Steve some boundaries, but she wants a "nice, friendly divorce." And besides, she and Steve still share lots of things, including credit card statements and their son Buzz. When Steve announces he had a fight with his girlfriend Ekaterina (the woman with whom he cheated on Mack) and needs a place to stay, Buzz offers the camp, and Mack can't bring herself to say no. Moments later, mayhem breaks out in the dining hall, as Cole takes the mic to announce that the camp's annual game of Capture the Flag, played over two days, is about to begin. Yellow and green teams are determined by the sticker under everyone's breakfast plates. Finding themselves on Team Yellow, Buzz and Steve resolve to grind Mack and her Green Team to dust.
Kip has just finished taking his leukemia medication when he's called out by camper Chloe. She knows all about his leukemia, having made it her business to know everyone's secrets, and Kip doesn't want anyone knowing his. Kip proceeds to the first meeting of the Green Team, which quickly chooses Mack as their leader, while Steve pushes Buzz and Sarah out of the way to take control of the Yellow Team. After explaining the rules, the team leaders head out to hide their flags. Cole comes down hard on Mack for letting Steve return to the camp. Sarah also wants to take Steve out too, for blowing in and taking control of everything. Just then, a kid runs up with a message that Robbie's mom called. Robbie doesn't want to call back, knowing his mom Sunny, a habitual gambler, has probably bottomed out again.
The whistle goes off, signaling the beginning of Capture the Flag - which means water balloon and super soaker mayhem. Kip runs off to read a book, but he can't escape Chloe. Smacking him with a water balloon, she makes him her prisoner, threatening to tell everyone he has leukemia if he doesn't do what she says. Chloe pelts Kip with questions about his leukemia, which he feels are entirely inappropriate. Meanwhile, Robbie finds his mom at the casino. She put all her rent money into her lucky slot machine, and she needs to get it back. But Robbie can't give her any money; everything he's got is going to pay his law school tuition - which Sunny can't understand - law school is so boring! When Sunny tears up, Robbie swallows his pride and borrows the money from Rodger, who tells him to cut his mother loose.
Cole rigs up a baby monitor under Steve's cabin, hoping to learn the location of the Yellow Team's flag. Mack is upset with Cole for cheating, until she overhears Steve asking Buzz to live with him half the year instead of just weekends. Irate, Mack runs into enemy territory. Steve admits he should have discussed Buzz's living arrangements with her first. After bonding over memories of Buzz, Steve leans in for a kiss, discombobulating Mack. Hearing all this over the baby monitor, Cole runs up, slings Mack over his shoulder and carries her off. Cole can't understand: Steve cheated on Mack, then tried to steal Buzz. Cole thought Mack was too smart to fall for such tricks! Calling him a burnout, Mack reminds Cole that he works for her, so he better go find something to fix that's not her life.
In the heat of battle, Sarah coaches Robbie on how to deal with his mother. Sarah thinks Sunny only said dumb stuff because she's sad. Even though she can't remember the words, Sarah advises Robbie to live his life according to the Serenity Prayer, promising everything will be okay. Mack has cocktails with Raffi and Todd, unhappily contemplating the prospect of losing Buzz to Steve for half the year. Todd reminds her that Buzz is going off to college soon; maybe the transition will be good? That night around the campfire, Kip is sad to learn Marina's hitting it off with her sexy teammate Greg. Mack tells Buzz that Steve wants more time with him. She's fine with it, as long as they can still have their Sunday nights together to watch "Game of Thrones." Moments later, Ekaterina shows up to make peace with Steve, throwing another monkey wrench into the works.
Later that night, Steve knocks on Mack's door to explain he got back together with Ekaterina. They're moving into a new condo with two bedrooms, and Ekat needs the second one for her Pilates machine, so Buzz needs to go on the back burner. Furious, Mack demands that Steve deliver the bad news first thing in the morning. Mack spies father and son sitting together at breakfast and figures Steve is breaking the bad news, but in fact, he's talking about breasts. When Kip spies Marina and Greg holding hands, he offers to tell Chloe the location of the Green Team's flag, in hopes of ending the game once and for all.
Buzz slams Grace with a water balloon, making her his prisoner. She offers a deal, which will only be good for 10 seconds: she'll show him her boobs if he lets her go free. Grace lures Buzz back into Green territory, than tags him with a water balloon, making him her prisoner. When Chloe delivers her prisoner to Green headquarters, Mack reluctantly tells Buzz the truth - living with his dad for half the year is off. Seeing Mack is furious with Steve, Sheila bribes her son with a candy bar, quickly learning the location of the Yellow Team's flag. Across the camp, Chloe tells the Yellow Team the location of the green flag, just as Robbie's called away - Sunny has another emergency.
At home, Robbie is stunned to learn his mother used the $700 he borrowed from Roger for more gambling. Having won $7,000, Sunny is elated. She set up a celebratory dinner, paid Robbie back with 100% interest, and bought him a present. Furious, Robbie leaves, telling his mother to do whatever makes her happy - but he can't take a ringside seat in her life anymore. Back at the camp, Kip confesses to Cole. He told Chloe the location of their flag so she wouldn't tell everyone about his leukemia - but the real reason he squealed was that he was mad at Marina for making out with Greg. The only good thing about the camp is that no one knows he's sick. Cole tells him to look at the big picture and let leukemia pay some dividends: cancer is what will get Kip laid this summer.
As the two teams converge on the location of the Green Team's flag, water balloon mayhem breaks out. Sarah tags out Cole. Just as she runs to capture the Green Team's flag, Robbie "kills" her with a water balloon. On the other side of the camp, Mack tackles Steve and sits on his back, furious about Buzz. Clearly Steve doesn't deserve a nice divorce - it's time for a nice, normal, angry divorce. Steve is going to tell Buzz the truth - that he screwed up - and then he's going to get out of Mack's camp! Mack grabs the yellow flag, and victory for the Green Team, and storms off.
That night, the Green Team thoroughly enjoys being served dinner by the Yellow Team, and Mack is happy to see Steve is finally talking to Buzz. She approaches Cole to apologize for their fight, especially for calling him a burnout; she knows he has her best interests at heart. They shake hands, but it's pretty clear Cole would like to shake a whole lot more. Chloe spies Kip leaving the dining hall, so she follows him. When he angrily tells her she can tell anyone she wants about his leukemia, she hauls off and kisses him, making Kip's day.
That night, Robbie tells Sarah about his evening with Sunny. After leaving her house in a huff, he finally opened her present, a Mont Blanc pen. Who uses those? Lawyers. So Robbie drove back to his mom's, told her he loved her, and took part in her celebration - even going so far as to blow on her scratchers quarter for luck. With his new Serenity Prayer outlook on life, Robbie is able to have fun with his mom.
Mack's ex returns to Little Otter, wreaking havoc during an epic game of Capture the Flag.