Editor's note: The air time of this episode has changed.

PENNY'S EX-BOYFRIEND, ZACK, ASKS LEONARD FOR A HUGE FAVOR, ON A REBROADCAST OF "THE BIG BANG THEORY," THURSDAY, AUGUST 8

Brian Thomas Smith Returns as Penny's Ex-Boyfriend, Zack

"The Propagation Proposition" - Penny's ex-boyfriend, Zack (Brian Thomas Smith)
, and his wife want a baby, but when Zack is infertile, they ask Leonard for help. Also, Koothrappali attempts to mend things with his ex-fiancée, Anu (Rait Gupta)
, on a rebroadcast of THE BIG BANG THEORY, Thursday, August 8 (8:00-8:31 PM, ET/PT) on the CBS Television Network. (Originally broadcast 1/10/19.)

REGULAR CAST:

Johnny Galecki (Leonard)
Jim Parsons (Sheldon)
Kaley Cuoco (Penny)
Simon Helberg (Wolowitz)
Kunal Nayyar (Koothrappali)
Mayim Bialik (Amy)
Melissa Rauch (Bernadette)

RECURRING CAST:

Brian George (Dr. Koothrappali)
Brian Thomas Smith (Zack)
Rati Gupta (Anu)
Lindsey Kraft (Marissa)

GUEST CAST:

Shay Ali (Greg)
Chip Chinery (Bob)

STORY BY: Chuck Lorre, Steve Holland, Jeremy Howe

TELEPLAY BY: Maria Ferrari, Dave Goetsch, Eric Kaplan

DIRECTED BY: Mark Cendrowski