Case Closed: Episode One - The Great Detective Turned Small(2016)

Meitantei Conan: Episode "ONE" - Chiisakunatta Meitantei

A special commemorating the 20th anniversary of the anime series. The special is a retelling of how Kudo Shinichi was transformed into a child.

It all started when Ran lost her smartphone. Shinichi promises to replace it, but the high schooler is already making his name as a consulting detective for the police; when he abruptly leaves her karate tournament to skillfully solve a high-society murder case, Shinichi's going to have a take Ran on a special date to get back on her good side. A colorful amusement park beckons — and so does a horrifying act of homicide!

(Source: Anime News Network, TMS Anime)