Naughty Dennis Mitchell, a mischievous five-year-old rascal, is dedicated to making his forbearing next-door neighbor George Wilson's life miserable, even though he really doesn't mean it. But now that school is out for the summer, he has all the time in the world to channel his overbearingly excessive energy to his beloved pensioner who only wants to be left alone with his garden. Meanwhile, a tattered thief visits the town. Undoubtedly, Dennis only wants to be of assistance. Who can hold a grudge against this innocent, adorable little monster?