Big Fat Alien Wedding is the forty-third episode of Ben 10.
The episode begins in the Rust Bucket where Ben is trying on a tuxedo at Max's insistence. Ben is not happy that he has to be in a wedding.The Tennysons arrive at the site of the wedding. Max's nephew and the groom, Joel, and Max's brother, Gordon, both want Max to talk sense into the other. Gordon says that the wedding is a terrible mistake. The bride, Camille's, parents arrive, and they are cold to Joel's parents. Joel tells Max that everything is going wrong. Joel tells Gwen that she won't be the flower girl because their first choice, Camille's cousin, couldn't make it. Ben learns that he has to dance at the wedding. Later, Gwen notices that Ben is missing. He heads down to the lake, but stops at a dessert truck. He starts eating a cake, but is caught by the person unloading the truck and leaves. A pile of mud grabs the man and drags him into the forest. A duplicate of him walks out. A sign by the lake says no swimming. A truck drives up and Ben transforms into Cannonbolt and crashes into it. The wedding stage lights on fire and Cannonbolt tries to put it out. He jumps into the lake, sending a huge wave onto the fire. Gordon and Betty Jean aim Plumber weapons at him, believing him to be a wedding crasher. Joel and Max arrive and Max defends Cannonbolt. Gwen casts a spell that creates a smoke. Gordon and Betty Jean start shooting, but Joel tells them not to ad makes them promise no weapons. Cannonbolt rolls away.In the parking lot, Cannonbolt is attacked by a pile of mud, and he reverts back into Ben. The mud pile slithers away. In the Rustbucket, Max says that Joel's parents were Plumbers and that Camille's family is a species of aliens called Sludgepuppies that has been feuding with the Plumbers for ages. He says that the wedding could end the feud, and that someone doesn't want that to happen. At dinner, Ben meets the flower girl, Lucy, and Gwen offers to teach him how to dance. The man from before transforms into a mud alien and attacks. He is Camille's ex-boyfriend. Ben transforms into Diamondhead. The alien grabs Joel. Diamondhead saves him and fights the alien. They fight in a spa, where Diamondhead sends the alien into a pool of water and he dissolves. Later, Gordon wants the wedding to be cancelled, but Camille refuses and Joel still doesn't want his parents to bring weapons. In the Rustbucket, Gwen teaches Ben how to dance. At the wedding, Camille's parents turn into mud aliens and attack the wedding.
Ben realizes that the Manns were behind everything. They grab Joel and attack Camille. Ben transforms into Heatblast and attacks them, freeing Joel. The Sludgepuppies and Heatblast engage fight, while a third sludgepuppy attacks the rest of the Tennysons. Max gives Gordon and Betty Jean weapons and saves Gwen from the sludgepuppy. Heatblast and the Manns fight in the forest and Camille, who is angry, transforms into a sludgepuppy and helps the Plumbers. Gordon and Betty Jean apologize for being rude to Camille. Max blows up the sludge puppy. In the forest, Heatblast hardens Mrs. Mann, but she fixes herself. The Manns merge into a giant mud alien and grab Heatblast. They suck him in, but he makes them so hot that they fully harden and breaks out. The wedding finishes and Ben is seen crashing some wedding pictures. Then the episode ends with a hand closing the wedding album.
Gordan: (Walking up with Betty Jean to Camille, looking guilty) We're sorry that we've been so tough on you, Camille. Betty Jean: Can you ever forgive us? Camille: (Looking at the two) Of course, that's what families are for. (Looks ahead at the bodyguard as she pushes them aside) LOOK OUT!
Camille's ex-boyfriend: (Holding Joel in his grip, looking at him angrily) If I can't have Camille, then neither can you. You meat puppet. Diamondhead: (After launching diamond shards to cut the Sludge's hand off, thus saving Joel) Back off, Dirt bag! Camille's ex-boyfriend: A Petrosapien? What are you doing here? Diamondhead: Uh, friend of the groom? Camille's ex-boyfriend: Then you're no friend of mine. (begins to attack)