Henry (Joe Morton) and Grace (Tembi Locke) want a small, quiet wedding but Jo (Erica Cerra), perhaps missing the wedding she and Zane (Niall Matter) never had, has more elaborate plans in mind. For Carter (Colin Ferguson) and Allison (Salli Richardson-Whitfield), their romantic attachment is developing only-in-Eureka complications: the Department of Defense has sent relationship auditor Warren Hughes (Wallace Shawn) to assess them. Things get off to a rocky start and any hope of making a good impression slips away when Tiny, returning from Titan, explodes on arrival and poisonous gases begin threatening the town. While Carter and Henry try to find the source of the contamination, Fargo (Neil Grayston) and Holly (Felicia Day) struggle to remain focused on their Astraeus training despite their growing attraction. The pair have decidedly different ideas about how to handle the situation and Zane's glib advice just adds to their frustration.
With Warren watching their every move, Carter and Allison work together to halt the deadly deterioration of Eureka's atmosphere, but it falls to another couple to help save the day. Fargo and Holly must get past their issues to execute a daring, dangerous mission -- in the midst of a toxic snowstorm.