Which Witch is Which? is the twenty-third episode of Season 3.
While preparing for a town festival for the foundation of Dimmsdale, Timmy travels to the past to see the foundation, but he finds himself in trouble when a phony witch hunter accuses him of being a witch and the town believes him.
Dimmsdale is holding a Founder's Day Parade and Timmy and A.J. are fighting over who they are going to model their float after, the legendary witch hunter Alden Bitterroot or the founder of Dimmsdale, Dale Dimm. A.J. contests that Dale Dimm did not even exist and he was just a folk legend, and that there would be a portrait of him if he was real. Timmy and A.J. Chester stops Timmy and A.J.'s fighting. make a deal that if Timmy can prove Dale Dimm existed, they would model their float after him instead of Bitteroot. While A.J. drags Chester to the library, Wanda appears and warns Timmy that he won't be able to "out-library" the genius A.J. But Timmy has a better plan and wishes he could go back to when Dimmsdale was founded.
Timmy and his fairies appear in "Ye Olde Town With No Name" in the year 1665. They run into Alden Bitterroot, who looks nothing like the muscle-bound hero he is portrayed to be in the twenty-first century, but rather a Colonial version of Mr. Crocker. He easily fools the townspeople into believing that a duck is a witch by dressing it in a pointy hat with a broom, and they decide to give it "the slow roast of justice". Meanwhile, Timmy has run into the slovenly and dumb Dale Dimm, who exists but is clearly the village idiot. Dale Dimm at first mistakes Timmy for a witch due to him being clean, but befriends him. When Bitterroot comes after hearing of another witch, Dale points to a log which Alden dresses in witch clothes and declares it to be a witch log. He then stabs the log with a pitch fork and takes it away to be roasted. A town naming ceremony is held, and after giving Alden Bitterroot two huge bags of gold, they also decide to name the town "Bitterburg" because of the two witches he caught. Timmy cries out against this, saying that the town should be named after Dale Dimm and that Alden Bitterroot was a total fake.
Alden Bitterroot accuses Timmy of doing witchcraft because he had crafted a sandwich out of the food there, something that was unknown at the time, and because he smells clean. Alden Bitterroot ties Timmy up to an anchor in drops him in the lake, saying that if he drowns he is human but if he survives HE IS A WITCH. Not having much time to protest the absurdity of that, Timmy instead wishes to be able to breathe underwater and rushes back to expose Bitterroot for the fake that he is. Meanwhile, the mayor is busy piling more money on top of Bitterroot when Timmy crashes the ceremony yet again, but is chased off when he fails to convince the townspeople Alden Bitterroot is lying. Cosmo is now the hat Timmy is wearing and Wanda is now the suit. Ready to give up, Timmy wishes himself away as he is being chased, but Cosmo and Wanda's magic isn't working. There is a real witch blocking their magic. Timmy decides that they must find the real witch to return to the present and he then notices that Alden Bitterroot has a broom and keeps a black cat, so he must be the witch. He then yells at Bitteroot that he accuses Bitteroot of being a witch, but shortly after saying that the villagers grab Timmy by the arm. Bitteroot claims that Timmy is mad, but Timmy slyly asks him "Oh then why don't your feet touch the ground?". It then shows that Bitteroot's feet aren't touching the ground which causes the villageres to finally realize Alden Bitteroot is the witch and release Timmy (With the mayor stating, "And many of us were a little perplexed about that."). Bitterroot reveals this to be true and transforms his hat into a witch's. He then turns on the villagers and begins flying around zapping green magic at Timmy... until he flies right into Dale Dimm's foot. Bitterroot tries to attack Dale Dimm again but is quickly stepped on, Dale Dimm then shakes the witch off his foot and drops it into the well. The townspeople thank Timmy, but Timmy tells them that Dale Dimm was the real hero and that they should name the town after him (They mistakenly declare the town will be called Dalesburg but Timmy states they should call it Dimmsdale). Timmy then gets them together for a photograph, which he uses to win the deal against A.J. he made earlier. A.J. makes a giant Dale Dimm robot which unintentionally stomps on the other floats. Timmy also changed history by turning it into Founder's and Invention of the Sandwich Day. Meanwhile, Alden Bitterroot is just managing to climb out of the well, despite being in there for over 300 years, but Mr. Turner throws a piece of his sandwich and it hits Alden Bitterroot who falls back into the well screaming.