When Walter finally let his guard down about his sister's death.
Losing Megan was hard on the whole team—Walter in particular. After losing his sister, the genius shut down and attempted to compartmentalize his pain. When his pet ferret escaped its cage, Walter lost his cool, puzzling Team Scorpion by throwing a fit and yelling that "he didn’t want to lose her." He broke down in tears after finally admitting how much pain he was in—and Paige immediately embraced him.
Watch "Dam Breakthrough" on CBS All Access.