Kenji to Mary no Tegami

"Dear Kenji, please help me. I'm in Osaka right now."

After school, Kenji, a sixth grader, found a letter in his classroom which reminded him of the urban legend "Mary's Phone Call". At first he didn't pay any attention to it, but after finding the second letter, he realized that it's not just a prank.

Who really wrote the letters? Is Mary real? In order to find out the truth, Kenji and his classmates team up.