"Serengeti Western" begins by introducing Timon, a meerkat, and his warthog friend Pumbaa. At the start of the episode, Timon and Pumbaa are hanging from two tree branches about to fall in a pit filled with snakes and scorpions. Timon blames Pumbaa for what happened earlier, causing the scene flash back to when the two friends take two trains to visit the old west.
Timon and Pumbaa are at a bar playing poker and after they win, three outlaws named Mad Dog McGraw, Billy the Goat, and Three-fingered Jackelope, show up to the bar. Mad Dog tells Timon and Pumbaa to meet him at noon.
Later, one of the residents of the west tells Timon and Pumbaa that Mad Dog is planning on taking over the town. Instead of fighting Mad Dog, Timon tries to sell the town to the people, but nothing works as they refuse to own it. Therefore, Timon and Pumbaa leave the old west and never be heard from again.
But then, Mad Dog and his henchmen show up and rob the train. Timon suggests that he and Pumbaa should fight Mad Dog after seeing that they can earn a $10,000 reward for doing so. The two put fake mustaches on their faces to disguise themselves as Mexican bandits and see what Mad Dog is up to by visiting his secret lair.
Mad Dog and his minions take Timon and Pumbaa to their lair, with the duo blinfolded. After Timon and Pumbaa get their blindfolds removed, Timon has his friend talk to the Jackalope about Mad Dog while he tries to distract Billy the Goat. Mad Dog sees that he was given bananas, which he is allergic to. The criminal dog then finds Timon and Pumbaa whispering, making him become suspicious and removes the mustaches from the duo's faces. Timon and Pumbaa get chased by the outlaw gang. They attempt to escape by falling into a ravine, but they still get caught.
The scene switches back to the present. Timon and Pumbaa fall of the tree branches but miss the pit. Timon states that the reason they survived is for them to save the old west by stopping Mad Dog.
It is now noon and Timon and Mad Dog are getting ready to fight. Timon takes out two bananas, causing Mad Dog to flee since he's allergic to bananas. So Timon and Pumbaa continue shooting bananas at the criminal and as they do so, Billy and Jackelope get caught by getting surrounded by bananas. Mad Dog locks himself into a prison and gives Timon and Pumbaa a key, symbolizing that they now own the town.
Timon and Pumbaa rejoice over saving the old west but then see that destroyed it with the bananas. This makes the town residents angry at the duo, making them run away with them giving chase.