Contains spoilers for this episode and earlier episodes.
A fly gets into the underground laboratory, and Walt refuses to resume work until it is caught. What begins as a concern about contamination occupies him through the night. He climbs onto the walkway, falls while trying to reach the insect and is still pursuing it when Jesse returns. Jesse wants to get on with their work, but Walt locks him out rather than abandon the search. Cutting the power gets Jesse back inside, and he brings flypaper and secretly adds sleeping pills to Walt's coffee, hoping exhaustion will finally give way to rest.
As they wait, their conversation turns to illness and family. Jesse recalls his aunt's cancer; Walt insists that his own remains in remission. Walt then tries to identify a moment when dying would have spared his family the damage he has caused. He settles on the night Jane died and tells Jesse about meeting her father in a bar. He apologises without revealing that he watched Jane choke and failed to help. Jesse says her death was nobody's fault. Eventually Jesse kills the fly and lets Walt sleep. Afterward, Walt warns that he cannot protect Jesse if Gus discovers missing meth. Jesse denies stealing, and Walt later wakes to another fly buzzing in his room.