Mo. Rev. Stat. § 301.120: License plates returned, when.

When the owner of a motor vehicle moves the vehicle to another state, he shall return the license plates to the director of revenue within ninety days or upon the expiration of the period of reciprocity granted by the new state of residence; or if the owner of a motor vehicle ceases to operate the vehicle in Missouri, he shall return the license plates to the director of revenue within ninety days.