'The Big Bang Theory' to Feature 'Young Sheldon' Actors in Upcoming Crossover Episode

The Big Bang Theory will be introducing Sheldon Cooper to Young Sheldon in an upcoming crossover episode.

The CBS comedy series will welcome Young Sheldon cast members Iain Armitage, Lance Barber and Montana Jordan for a new episode, set to air in December. The actors will play their Young — Sheldon, George and Georgie, respectively — during the episode.

No details on plot for the episode or what will find Sheldon seeing images from his past have been revealed yet, Deadline reports.

Young Sheldon co-creator Steve Molaro previously opened up about the possibility of both shows getting the crossover treatment back in April.

"There's no reason why we can't keep going back and forth in this weird time portal that has been created between the two shows," as Young Sheldon is set three decades before the events of Big Bang.

The flagship series has made several nods to the spinoff series in recent episode, including ahead of its season 11 finale when Sheldon's brother Georgie was finally introduced — played by Jerry O'Connell — just before Sheldon (Jim Parsons) and Amy's (Mayim Bialik) wedding.

The series also introduced the adult version of Sheldon's childhood best friend Tam in a recent episode. The character, who is played by Ryan Phuong on Young Sheldon, was introduced in his mid-40s played by Robert Wu. The episode saw Sheldon imagining his life had Tam moved to Los Angeles with how it was originally planned, though he changed his mind just before the move because he had fallen in love.

Producers have also expressed interest on showing the adult version of Billy Sparks, Sheldon's childhood bully, though no plans have been announced if it might happen during the final season.

CBS and Warner Bros. TV announced ahead of the season 12 premiere that Big Bang would be ending in May 2019, though executive producer Steve Holland revealed recently the writers are not exactly sure how the series will wrap up.

"I don't know exactly what the finale is, so I can't make that promise, but I think we've always felt this show is a happy, positive show about people who love each other, so we're not looking to kill everyone off in the finale," Holland told USA Today recently "You have to find a way to end this show that feels real to these characters."

The Big Bang Theory airs Thursdays at 8 p.m. ET followed by Young Sheldon at 8:30 p.m. ET on CBS.