California Code, Code of Civil Procedure - CCP § 993

If the defendant, in his answer, denies the judgment, or sets up any defense which may have arisen subsequently, the summons, with the affidavit annexed, and the answer, constitute the written allegations in the case; if he denies his liability on the obligation upon which the judgment was recovered, a copy of the original complaint and judgment, the summons, with the affidavit annexed, and the answer, constitute such written allegations, subject to the right of the parties to amend their pleadings as in other cases.