Let's say Better Call Saul runs six seasons. In season five, he meets Jesse Pinkman and gets him out of a jam in a corrupt manner, alerting Jesse to how he is a "criminal attorney.' ....In the last scene of the season finale, he opens the door and meets WW. Screen fades black.
Season six then starts in the Nebraska Cinabon, and we are now fully in the post-Walter White world. We see his life is useless, and he is almost suicidal. One night, he goes to take the trash out and guess who's standing there, angry and smoking a cigarette? None other than Skyler White. BOOM!
Turns out, Walter Jr. has spiralled down to drugs, and has gotten hooked on Baby Blue meth. But not only that, he's gotten ill with with some strange new illness. Skyler has with her a spiral notebook from Walt in which he predicted such an illness. She goes to doctors and authorities, but nobody listens. She runs into Ewell, the big black body gaurd who tells her Saul could help. He hooks her up with the Cleaner, who she murders to get Saul's info.
"Saul - you have to come back to Albuquerque and help me alert the media and the public about how baby blue is causing this illness. You've seen ask these deaths across the southwest on CNN. And now there's rumours that these deaths are something more, that these people are coming back as the undead.
Over a couple episodes, she convinces him and it becomes a redemption story for Saul. He returns to Albuquerque with Skyler. After only limited success in Albuquerque they set out for the CDC with samples of Baby Blue, and get pulled over outside of Atlanta by a rude, mean sheriffs deputy named Shane who shoots them dead after discovering the meth and them desperately scuffling with him, with Rick showing up too late to stop it.
....Skyler and Saul die. Cue Fear the Walking Dead. Fade to Black....