The Rikkai University-Affiliated Middle School's Boys' Tennis Team stumbles upon a strange bottle. Inside, a cryptic message asks for the recipient to add another item to the bottle and toss it back into the river. Having broken the container to read the note, the team puts it back together with tape and includes a few extra items before sending it on the long journey back to its owner. However, the bottle ends up washing ashore elsewhere, and the tennis team from Seishun Academy finds it instead.

[Written by MAL Rewrite]