When Paige literally breathed life into him.
When Walter and Paige were trapped in the Chernobyl power plant with their only exit blocked by a downed plane, the genius came up with a solution: scale the building to get out via the roof. As they reached the top, Walter dropped his oxygen tank and it tumbled out of reach. Given his depleted oxygen level, he couldn't gather enough strength to pull up on the final beam.
Paige took in her tank’s last breath of oxygen, lowered herself down, and locked lips to give Walter just enough strength to summit the beam.
Watch "Chernobyl Intentions" on CBS All Access.