A schoolgirl named Mikako Nagamine (Mika Shinohara (original Japanese), Sumi Muto (Japanese) Cynthia Martinez (English)) is recruited into the UN Space Army to fight in a war against a group of aliens called the Tarsiansnamed after the Tharsis region of Mars where they were first encountered. As a Special Agent, Mikako pilots a giant robotic mecha called a Tracer as part of a fighting squadron attached to the spacecraft carrier "Lysithea". When the "Lysithea" leaves Earth to search for the Tarsians, Mikako's friend Noboru Terao (Makoto Shinkai (original Japanese), Chihiro Suzuki (Japanese) Adam Colon (English)) remains on Earth. The two continue to communicate across space using the e-mail facilities on their mobile telephones. As the "Lysithea" travels deeper into space, messages take increasingly longer to reach Noboru on Earth, and the time-lag of their correspondence eventually spans years.