Tortoise John was the main antagonist of Rango. He was the wheelchair-bound tortoise mayor of Dirt, where there was a major water shortage.
From the very start, Tortoise John is made out to be very untrustworthy, greedy, and corrupt (such as keeping a large personal supply of water for himself), although he appears to be grandfatherly toward Rango.
When he first appears after Rango kills a hawk and defeats Bad Bill, he appoints Rango as the new sheriff and mentions that "whoever controls the water controls everything".
Later in the movie, it is shown that Tortoise John is building a modern city over the desert surrounding Dirt, and that he's been intentionally cutting off the town's water supply, which is actually connected to Las Vegas, and making the people believe that there's actually a drought, intending to let the town die out to make way for his new plans.
Tortoise John murders the town bank-keeper Merrimack by drowning him and when Rango starts to deduce his schemes, he hires Rattlesnake Jake to drive Rango out of town; but he comes back to stop them after fully discovering the mayor's plan. He captures Rango and Beans and starts to drown them in the town's water supply bottle. He then later betrays Rattlesnake Jake by pointing Rango's gun at him, telling him that he's also part of the obsolete Western era that he intends to destroy; but when he pulls the trigger, he finds that the last bullet was taken by Rango, who uses it to break open the water bottle, flooding the town hall and sending them all outside. There, Rango finally manages to subdue Tortoise John and he turns him over to Rattlesnake Jake, who then acknowledges Rango as another great desert legend worthy to save his life. With Tortoise John at his mercy, the infuriated Rattlesnake Jake paraphrases the mayor's previous words to him that "no one will believe you even existed", and he takes Tortoise John away into the desert to kill him for his treachery.