Hoshi no Koe (ほしのこえ) is the shortest anime I have ever watched. Only one 25-minute OVA, that was released in DVD on February 2, 2002, and later broadcasted on television. But the quality is not proportional to its length. In fact it is incredible how a so short anime can contain so many thoughts, feelings, things to learn, think.
A middle-school girl named Nagamine Mikako is recruited to the UN Space Army in a war against a group of aliens called the Tarsians, named after the Martian region (Tharsis) where they were first encountered. As a Special Agent, Mikako pilots a Tracer, a giant robotic mecha as part of a fighting squadron attached to the spacecraft carrier Lysithea.
When the Lysithea leaves Earth to search for the Tarsians with Mikako on board, Mikako’s friend Terao Noboru remains behind. The two continue to communicate across interplanetary, and eventually interstellar space via the email facilities on their mobile phones.
As the Lysithea travels deeper into space, the emails take increasingly longer to reach Noboru on Earth, and the time-lag of their correspondence eventually spans years.