Doug's a Big Fat Liar is the first part of the ninth episode of season two of Nickelodeon's Doug.
Doug wants to go to the Hoe-Down dance with Patti. But when Connie asks him, Doug lies and says he's staying home to take care of his sick cousin "Melvin." His lie gets him into big trouble when Patti and Connie come over to visit his cousin. Doug asks Judy to help him out and does such a good job because then, Melvin is invited to the Hoe-Down.
Doug's Journal Entry: Dear Journal. It was the day before the Bumpkin Day Hoe-Down. Well hat's the one dance of the year where the girls have to ask out the boys. Bet you can't guess who I was hoping to ask me. Her initials are Patti Mayonnaise.
At the hallway in school, Doug is expecting Patti to ask him out at the Bluffington School Hoe-Down dance when she walks to him with Beebe. But Connie asks him instead and Doug tries to come up with something to tell her.
Doug lies to Connie and tells her that he has to take care of his sick cousin "Melvin." Patti walks up to him to ask him out to the Hoe-Down. But Connie reminds her that Doug is taking care of his sick cousin. So Patti attempts to asks someone else to the Hoe-Down.
Doug's imagination: At the Hoe-Down, Patti is surrounded by the other guys, saying that she is glad that they are going with her. She also says that she feels sorry for Doug not making it due to taking care of his sick cousin Melvin. But Roger shows her Doug's family tree, which reveals that Melvin doesn't exist. Patti then realizes Doug's lie and gets furious.
While he plays Barnyard Chess with Skeeter at home, Doug then goes in a state of panic when he feels that everyone will catch on to his lie and soon, Patti and Connie come over to Doug's house to visit his "sick cousin." Doug slams the door and he and Skeeter have Porkchop impersonate "Melvin" for them. Porkchop lies in Doug's bed pretending to be sick. Judy overhears Doug and Skeeter talking about this and Porkchop walks away, not wanting to be involved in Doug's lie. So Judy tells Doug and Skeeter that there will be a Melvin.
Theda lets Patti and Connie in the house and as the two walk upstairs. Doug then decides to just tell them the truth, but the girls continue to walk upstairs and make it inside Doug's room, where "Melvin" is lying in bed sick. After a long moment of moaning, "Melvin" becomes cure and feels robust as "he" sings. Things get worse for Doug when "Melvin" agrees to going to the Hoe-Down with Patti. As Patti and Connie leave, "Melvin" is revealed to be Judy in disguise as she is eager to go to the Hoe-Down, much to Doug's dismay.
Doug's imagination: Doug is at a sub-committee to uncover big fat liars, with Doug's parents, Mr. Bone, Ms. Wingo, Mayor White, Roger, and Stinky as the interrogators. Mr. Bone asks Doug if he made the contention of having a cousin named Melvin and compelled Judy to play the part. Doug tries to convince him that it isn't entirely true. But Mr. Bone tells him that he has "liar" written all over his face...literally.
At the Hoe-Down, the others are less than suspecting of "Melvin's" attention after an hour, and Patti dedicates the evening to "him." As "Melvin" gives everyone "his" speech, "he" dedicates it to Doug, telling everyone a touching anecdote of the two's history. During "his" speech, however, the cow eats "his" wig and Judy is exposed to everyone. Doug then confesses his lie to everyone and Mr. Bone demands everyone to continue having fun at the Hoe-Down. Doug apologizes to Patti, who forgives him and dances with Judy. He also apologizes to Connie for lying to her and Connie tells him every lie she ever told in the past in return.
Doug is later in his room writing in his journal about how wrong lying is, adding that it's a lot more work than just telling the truth - unless the person is Judy, who actually plans to make a career out of it. Judy is later seen dressed as a pirate chasing Porkchop, who is tied up with rope, prompting Doug to yell at her in annoyance.