Miho, a high school student, refuses to approve of her widowed mother's potential husband when they meet each other at a restaurant. As she flounces out of the restaurant, she drops her cell phone. At the moment there is an earthquake, and she drops her cell phone down a staircase. The phone is found by Tokijiro, a boy that lives in 1912
Both movies have main characters who communicate with each other while living in different times. Communication in one leads to friendship but in the other leads to romance.
Both male leads are in contact with someone from the past and exchange pieces of information with each other through a specific machine. The big difference between these two is that they have completely different genres. Signal has little to no romance and focuses more on mystery suspense and crime while ditto focuses more on romance. Signal IMO is an outstanding drama, please do watch it if you have time.
Minato is a young girl who was traumatized at a young age by being abandoned by her parents and left with her senile grandmother. She frequently corresponds with a pen-pal named Night, a boy about the same age as her that she's never actually seen in person. Although Minato and Night are very different - Minato is upbeat while Night is brooding, they get along anyway and she regularly updates him on the happenings in her daily life. Minato is even willing to confide in Night that she's falling in love with a boy she's recently met named Sho.