Simplistically, assign a numeric value to every letter in a story. Add the numbers up. Compare that to what other stories add up to.

It sounds like that would be very slow, but you don’t let the letters get that big by taking a modulus (the remainder after division by some number). So if we always used mod 100, the numbers would range from 0 to 99. (We’d use larger numbers in real life)

If this story gets 57 and yours adds up to 32, they aren’t similar. If they both add up to 88, they might be the same story.