After an episode last week that, for the first time, explored what happened when a decades-missing guard returned from … somewhere … Fox’s “Alcatraz” returned to its returning-prisoner-of-the-week format tonight with “Paxton Petty.”

Petty — like the other 300-plus Alcatraz prisoners and guards who disappeared in 1963, only to return in the present day without having aged a day — shows up in modern-day San Francisco and returns to his old hobby. In Petty’s case, that means planting and exploding land mines.

Houser (Sam Neill), Madsen (Sarah Jones) and Soto (Jorge Garcia) track down Petty with relative ease, but the twist this week is that Houser, head of the special task force tracking down the returning “sixty threes” and a San Francisco cop back in the day, gets captured by Petty and stuck on top of a land mine. Madsen and Soto must rescue him.

The show mythology subplot this week, though, is more intriguing. Dr. Lucy Banerjee (Parminder Nagra), a modern-day expert working on the case, has appeared in several flashbacks to the 1960s and the show seems to be implying that Banerjee — like the mysterious and sinister Dr. Beauregard (Leon Rippy) — was one of the missing “sixty threes” and has now returned.

The show is slowly building its mystery, teasing us with what Houser and other characters who bridge both eras know. Eventually we’ll know as much (or little) as Madsen and Soto.

“Alcatraz” has potential, and very well might be working toward the level of mind-teasing drama that “The X-Files,” “Lost” and “Fringe” eventually reached.