The Salamancas and the cartel
8 episodes of Breaking Bad carry this thread, 8h in all — 8 of them are in The Essentials.
In airing order
Walter and Jesse do business with a new partner-in-crime, but immediately run into "production problems," which necessitates a nearly impossible mission to knock off a chemical-supply house. Meanwhile, Marie throws a baby shower for Skyler and presents her with an unusual gift that shows a different side of the aunt-to-be.
Season 2 kicks off with Walt and Jesse once again bearing witness to Tuco's incendiary disposition, which leads them to the conclusion that it's time to put an end to their business dealings with him. But the question is how. Meanwhile, Hank serves as a go-between for estranged sisters Skyler and Marie.
Trying to dissociate themselves from the volatile Tuco takes a bad turn when Walt and Jesse get trapped with the dealer, and Skyler, worried about her missing husband, receives comfort from her sister and brother-in-law. Hank, meanwhile, pays a business call on Jesse's mother.
In the third-season premiere, Walt faces a new threat on a new front and deals with an increasingly angry Skyler, who must consider what to do next with her life and the kids'. Meanwhile, Jesse comes face-to-face with some startling self-revelations.
Hank's behavior becomes increasingly more unpredictable and volatile, which leads to a confrontation with Jesse and problems at the DEA; Skyler wants Walt to make a deal; and Gus' decisions result in some serious consequences.
The Cartel makes a power move; Skyler and Walt reveal something embarrassing to the family; and Jesse's misdeeds bring some unwanted attention.
The family becomes gravely concerned when Walt doesn't show up for his son's 16th birthday celebration. And Jesse's lab skills are tested in Walt's absence.
The fourth season ends with Walt and Jesse squaring off against Gus. Elsewhere, Walt acquires a surprise confederate with help from Saul.