The objective of this invention is to provide an image synthesis method, games machine, and information storage medium that make it possible for a game to be satisfyingly enjoyable, even when a single player is playing it. History information relating to the performance of the player is recorded during one game session, and this history information is reproduced in another game session. A virtual player performs based on the thus-reproduced history information, and thus acts so as to affect the game result of the player. The virtual player either assists the player or acts as a combat opponent. This makes it possible to implement a game that is interesting even in single-player mode.