After the end of Malcolm in the MIddle, the mom, Francis, Reese, Malcolm, Jamie and the other child they presumably had afterwards all died, leaving just Dewey and Hal.
With Hal constantly switching careers and failing at each one, he was unable to financially support himself and his son. With this, he became involved with loan sharks whom he borrowed money from and was unable to pay them back.
As a result of this, Hal decided to escape his debt by disappearing from his old town, changing his name to Walter White and Dewey's to the same and worked as some chemistry professor or something, I don't know. I don't care and that's, like, the beginning of Breaking Bad. He also got remarried.