Nathan Stark is a Nobel Prize-winning mathematician, Eureka's head researcher and Allison Blake's estranged husband.

Charming to a fault and keenly brilliant, he gives Jack Carter a run for his money in the contest for Allison's romantic interest, but Stark's professional desire to push the boundaries of science eventually compromise his personal integrity.

Stark is still in love with Allison, which explains why he has not yet granted her a divorce. Carter's arrival in "his town" and budding relationship with Allison sets up Stark as Carter's bitter rival.