The episode begins and everyone except Barney seems to be hiding in a closet, and we see each of them on their cell phones talking to each other, with Marshall telling the others he just saw the worst thing ever while the others worry that "he" can hear them. One week earlier, Marshall and Lily are telling Ted and Robin that Marshall's mother is coming to town and they need to figure out a place for her to stay. Robin is questioning whether this Barney-Patrice thing is for real, thinking Barney is trying to get back at her for rejecting him. Lily suggests to Marshall that his mother stay with Ted, but Marshall says no.
Robin comes to the apartment, annoyed that Barney came to the office to take Patrice to lunch. Robin says they need to keep trying to stop this thing from happening, and then admits that she's been trying to tell Patrice who Barney "really is". Robin then yells at Patrice for not believing her. Robin says Barney is using her. She wants to show Patrice the playbook that Barney uses. Ted tries to convince her, this obsession isn't right, and she needs to let it go. Robin thanks him for talking some sense into her. Next she is seen breaking into Barney's apartment. Marshall's mom has arrived and tells Lily that she's been thinking about getting back into the dating world. She wants some advice from Lily.
Lily encourages her to "let the girls out to play," and opens up the buttons on Judy's shirt. Robin gets into the apartment and snoops around for a bit. Barney gets home and Robin hides in a closet. Barney tells Patrice to get over there and says, "There's not one reason to leave this apartment tonight". Robin calls Ted from her cell phone in Barney's closet. She asks him to help get her out and in return she offers to also go with him to a five-day fan festival in the desert. Barney decorates his tree with Ted's ornaments when there's a knock on the door. It's Ted, saying Hugh Hefner is in the lobby of the building.
Barney runs to grab his robe and pipe. Robin gets out of the closet, and then continues to snoop around. She trips over Barney's Storm Trooper and another secret hiding place is revealed. Down comes a copy of "The Playbook". Robin takes it. Marshall is in shock on finding out his mother is sleeping with Mickey. "You've been been bumpin' uglies with my daughter for years" Mickey says. "The second I climb up on your mom, it's the end of the world?" he adds. Marshall runs to the bathroom to puke. Robin hears Barney and Ted coming and races back to the closet. She immediately calls Ted back and asks for help again, but he refuses. "What if I told you I was looking at your red cowboy boots right now?" Robin asks. "Describe them" Ted says, slyly. "They're red and they're cowboy boots" Robin replies. "That's them" Ted says, wondering how they got there.
It is then shown that Stuart actually returned the boots to Barney at the bar. Ted agrees to help after Robin threatens to cut the boots with a pocket knife. He again tries distracting Barney, but to no avail. Barney closes the door on Ted, who sneaks back in after Barney turns his back. Ted ducks into a different closet when Patrice shows up and Barney goes to answer the door. Ted calls Robin to tell her he got her purse, but he's stuck in a different closet. Robin decides to call in Lily for help. She dials Lily and hears a phone ringing. Robin looks out of the closet she's in and sees that Lily is hiding in another closet in Barney's bedroom. They each ask what the other is doing there, but Lily has to go because Marshall's calling her on the other line.
Marshall has called Lily to tell her he just saw the worst thing ever. Lily and Robin are both hiding in Barney's bedroom closets while Ted is in the living room closet. Lily runs over to the closet Robin was hiding in before Barney returns to the bedroom, and they hide together. Lily explains she's been using Barney's apartment to pump breast milk because her apartment has gotten so crowded. She uses a cooler that she'd borrowed from Ted and never returned. When Lily heard Robin breaking into the apartment, she went and hid. Robin tells Lily her reason for being there. Barney asks Patrice to go to the bedroom to grab his computer because he wants to change his Facebook status to "happy". Lily tells Robin she shouldn't have The Playbook in her hands, so Robin runs out of the closet and puts it on Barney's bed.
Patrice finds it and asks what it is. Barney admits it's the book of trick he's used to get women to sleep with him. He shows her one, and she gets angry and prepares to leave. They go out on the balcony to argue over it, and Robin and Lily try to make their exit, but only make it to the living room and end up hiding in Ted's closet. Patrice tells Barney she can't be with a man who has a playbook, so Barney tosses it in the trash and sets it on fire. Patrice is happy, telling Barney she knew he was a keeper. Barney puts out the fire when the smoke alarm goes off, then leaves to tell the super. Patrice goes with him.
Lily gets home and tells Marshall that he has to let his mom "get out there and rediscover her sexuality with someone special". "My mom and your dad are humping," Marshall tells her. Lily pukes on hearing this. Barney says they should be happy for them. He goes on to say that "sometimes you go for someone you'd never expect, but doesn't everyone deserve to be happy?" The whole group is stunned as Barney goes to the bar buy another round. Robin thinks Barney's lost and needs an intervention. She thinks Barney's behavior is a cry for help.
She says that if she were this out of control, she'd want them to have an intervention for her. "So, what do you say? Intervention?" Robin asks. They all agree. Later, Robin show up for what turns out to be an intervention for her. Marshall and Lily apologize to Judy and Mickey for the way that they acted when they found out about them. Next Marshall and Lily are shown shivering in their own closet, jointly puking. The episode ends at this point.