Missouri Revised Statutes Title XXXV. Civil Procedure and Limitations § 517.031. Petition required, form--affirmative defenses, counterclaims, cross-claims, filing--extension of filing, when

1. The plaintiff shall file a written petition containing the facts upon which the claim is founded. A copy of any written instrument or account in support of the petition should be attached and filed. The pleadings of the petition shall be informal unless the court in its discretion requires formal pleadings.

2. Affirmative defenses, counterclaims and cross claims shall be filed in writing not later than the return date and time of the summons unless leave to file the same at a later date is granted by the court. No other responsive pleading need be filed. If no responsive pleading is filed, the statements made in the petition, affirmative defenses, counterclaims or cross claims shall be considered denied except as provided in section 517.132.

3. For good cause shown, the court may extend the time for filing any pleading.