This study evaluates an original game, Sea Monkeys, which contributes to the field of adaptive games while testing the viability of exergames that present continuous, expandable narrative. Adaptivity stands to assist game designers in resolving challenge management, a sophisticated problem in exergaming. Psychological flow provides a unified perspective from which to evaluate experiences of physical exertion and game engagement. The study was performed using independent measures to discern the effect of adaptivity (n=25). Participants were exposed to two rounds of the game using full-body motion input and equipped with a heart rate monitor. Cardiovascular exertion meeting exercise standards was observed. Measurements suggest that adaptivity enhanced the reliability of the challenge encountered by players. Players reported the narrative to be engaging and helpful, and flow experiences were indicated. Sea Monkeys demonstrates an engaging, effective, and expandable game design for delivering exercise benefit.