When Nicky, Ricky and Dawn discover that Britt has been hanging out with their friends without them, they start to question his motives.
Dawn, Nicky and Ricky are watching spy movies with Britt. Their friends couldn't hang out with them because somehow they all had doctor appointments that night. By the time they're done watching, the quads realize that Britt is gone. He comes in later with Mae, Avery, Natalie and Miles. The quads are disappointed that their friends ditched them and are instead hanging with Britt.
Their friends all claim that their doctor appointments were in the same building. And so, they decided to hang out together afterwards and enjoy root beer floats. Britt says that they all got group texts to hang and he didn't want to interrupt the quads. So, he left without telling them. As Britt goes up to his room and their friends leave, the quads become suspicious that Britt is lying. Their fears are confirmed when they realize that they didn't get any group texts and that it was in fact Britt who initiated the group chat.
The quads conclude that Britt doesn't want to hang out with them. Dawn thinks that Britt is trying to avoid Nicky and Ricky. But the boys think he's trying to avoid Dawn. So, the next evening, they all separately try to trick Britt into hanging out with them. Britt lies to Dawn that he's hanging with Nicky and Ricky; and lies to the boys that he's hanging with Dawn. This leads to a two hour standoff between Dawn and her brothers until they realize Britt lied to them. The quads track Britt and their friends' phones only to realize that they're hanging out together again. They come up with a conspiracy theory that Britt is trying to steal their friends. They agree to work together to prove their theory.
Using inspiration from the spy movies they've been watching, they agree that one of them should dress up seductively to lure Britt into admitting what he's up to. Dawn wants to do it because she's the only woman there but Nicky and Ricky have reservations. Dawn's attempt to look sexy and seduce Britt goes horribly wrong because she can't walk in heels and keeps falling. The experience is so embarrassing that Britt goes to his bedroom, leaving his backpack behind. Dawn grabs the bag and finds a note that lists Britt's plan. Britt returns and tells them that he's planning a surprise birthday party for Avery. However, in the note, Britt lists various items like get rid of the quads; take over Tom's Get Sporty and celebrate. After seeing Britt being too close with their parents, the quads become convinced that Britt isn't who he is and he is in fact trying to steal their lives.
So, before Avery's fake birthday party, the quads kidnap Britt and lock him up in a small room. They hijack the party to warn their friends about Britt. They set up a live video feed of Britt so that they can force him to admit what he's up to. They "torture" Britt to confess by covering his toes with peanut butter and having Squishy Paws lick them. Britt admits that he wasn't actually planning a birthday party for Avery. He says that he was secretly working with the quads' friends to throw them a surprise party for being so welcoming to him. He explains that he wanted to get rid of the quads and take over Get Sporty so that he can throw them the surprise party there. The quads are impressed by Britt's gesture. They realize that they let their conspiracy theory get out of hand.
- Lizzy Greene as Dawn Harper
- Casey Simpson as Ricky Harper
- Aidan Gallagher as Nicky Harper
- Allison Munn as Anne Harper
- Brian Stepanek as Tom Harper
- Kyla Drew Simmons as Mae Valentine
- Jonah Hwang as Britt
- Siena Agudong as Natlee
- Theodore Barnes as Miles
- Isabella Revel as Avery
- Lou Ferrigino Jr. as Jett Masterson
- After mentioning it's her birthday, Avery blows the kazoo the same way she did after reading birthdays in The Buffa-Lowdown.
- Britt throws a party for the quads for letting him to start living with them in Quadcodile Dundee.
- This episode premiered alongside the Season 4 and the show’s finale episode, Lasties with Firsties; on August 4, 2018.
- Quadspiracy is a play on conspiracy.
- This episode featured Jett Masterson who hadn't appeared on the show since the episode, Go Hollywood.
- This episode marked the final episode title to have the word Quad featured in it.
- Britt makes his third and final appearance in the season and series.