Ernest P. Worrell, a well-meaning but accident-prone maintenance worker at a local bank, is selected for jury duty after inadvertently saving the life of a judge. During his time on the jury, he becomes convinced of the defendant's innocence, but his fellow jurors are more interested in getting the trial over with quickly.
Meanwhile, crime boss Mr. Nash and his henchman plan a bank robbery, but the two are nearly caught by the police and must find a way to escape. Nash's lawyer, who happens to notice Ernest's resemblance to his client, arranges for the two men to switch places. Ernest is sent to prison to serve Nash's sentence, and Nash takes over his job at the bank, giving him access to inside information and allowing him to plan the robbery from the inside.
While in prison, Ernest is mistakenly put into the electric chair and is given a massive shock. Instead of killing him, however, the shock gives him superhuman strength, electrical powers, and an alter ego named "Electro-ernest."
With his newfound abilities, Ernest breaks out of prison and teams up with a no-nonsense female bank worker to stop Nash's robbery and clear his name. Along the way, there are plenty of physical gags, pratfalls, and wacky humor typical of the Ernest franchise. Ultimately, Ernest saves the day, Nash is apprehended, and Ernest is hailed as a hero.