Mo. Rev. Stat. § 524.060: Petition — necessary averments.

It shall be sufficient for the plaintiff to aver in the petition that on some day therein to be specified, he was entitled to the possession of the premises, describing them, and being so entitled to the possession thereof, that the defendant afterward, on some day to be stated, entered into such premises, and unlawfully withholds from the plaintiff the possession thereof, to his damage, in any sum he may claim.