Klaus' journey is a rollercoaster of a ride, from his creation of a cult to his attempts to stop the love of his life from signing up for the Vietnam war. Unfortunately, he isn't able to do the latter; some events are fixed in time, and apparently, Dave's path to war is one of them. His failure with Dave causes Klaus to relapse and drown his sorrows in alcohol, but his siblings snap him out of his inebriated daze in time for him to join the mission to get them back to 2019.
Klaus, who unintentionally started the cult while abusing his connection to Ben to con people into believing he was a prophet, runs away from his followers to escape their clinging codependence. But when they learn the Hargreeves brought the apocalypse with them to the '60s, Ben convinces him to expose himself as a fraud to his followers. They don't believe him and refuse to leave his side, but this proves helpful when Klaus allows Ben to possess his body and confess his feelings to a young woman he's had feelings for since she joined the cult. Eventually, the pair have to leave the cult behind to team up with Diego and Allison to save Vanya and stop her from setting off the nuclear war. The battle winds up overwhelming Ben's connection to the living world and he ascends to the next plane, leaving Klaus without his constant, ghostly companion.
Later, Klaus travels with the family to Sissy's farm to save Harlan. He survives the subsequent attack from the Handler, Lila, and the Commission agents, and is with the group when they all time travel to 2019.