The shack has a party thrown by Mabel while the adults are away. Bill and Dipper are 13.
Mystery shack
"Hey kids I don't mean to be a downer but could you not have a party while I'm gone I'm still cleaning up after the last one." Stan told the kids as he and Ford are leaving.
"You heard him no parties understand?" Ford asked the kids.
"Yes." Mabel, Dipper, Wendy and Soos all said at the same time. Stan and Ford give each other a look and drive away
"Welp better get to work." Soos started but Wendy grabbed his arm stopping him.
"We have to throw a party." Wendy smirked mischievously getting a nod from Mabel.
"What no way are we having a party!" Dipper disapproved then jumped hearing Bill's voice body frozen.
"Hey Pinetree!" Bill bicycled next to Dipper "Did I hear you say something about a party? I love parties!" He said making her sweat "Are you okay?" He asked noticing her sweat.
"Yeah! Sure I am! And yes we are planning on throwing a party!" Dipper said "Today later at the shack but if you can't make it?" She hoped he can't make it but secretly hope he does so she's torn.
"Yeah it'll be fun." Bill said putting a hand on her shoulder smiling warmly then bicycled away.
"Awe Dipper! Your so awkward!" Mabel said hugging her twin.
"Well I guess were having a party even though the two Stans said no." Soos said matter of factly.
"I know isn't that awesome!" Wendy said ignoring her friend's concerns.
Later
The party was in full swing every teen in twin was there having a rockin' time listening to Soos DJ-ing and Mabel being the life of the party wearing her 80s outfit while Dipper is wearing a blue dress and bow sitting next to the lunch bowl.
"How is everybody enjoying the teen only and no adults allowed party?" Wendy asked holding a microphone she gets cheers in response.
"Hey Dipper." Bill said in a suit "Wanna dance?" Be asked hand out which Dipper takes.