Fans of "The Big Bang Theory" can look forward to seeing Sheldon and Amy reach a new point in their relationship and meeting Howard Wolowitz's long-lost brother in Thursday's all-new episode.

A preview of the next episode, which is entitled "The Fortification Implementation," reveals that Sheldon and Amy will build a fort in his apartment. However, once they're alone in the fort, Amy suggests that the two share their very first "sleepover."

In addition, viewers will be introduced to Howard's younger half-brother, Josh, played by "Victorious" star Matt Bennett. Josh, who shares the same father with Howard, will unexpectedly come to town and show up on his brother's doorstep. Howard's estranged father, however, has yet to appear on the series. Bennett has also starred in "Bridesmaids" and "iCarly."

Actor Wil Wheaton will also make a comeback in the upcoming episode, teases Carter Matt. Plus, director Kevin Smith will have a voice-over cameo.

The synopsis reads as follows:

"When Sheldon and Amy build a blanket fort, they come to major crossroads in their relationship. Also, Wolowitz meets the half-brother he never knew he had, and when Penny is invited to appear on Wil Wheaton's podcast, she and Leonard have a huge, on-air argument."

Later this month, Sheldon and Leonard's mothers will return to the show, but this time in the same episode, TVLine reports. "McCarthys'" Laurie Metcalf will reprise her role as Mary Cooper while "The Good Wife" star Christine Baranski will return as Beverly Hofstadter during the penultimate Season 8 episode on April 30. Baranski last appeared on the series as Leonard's mom in the Season 7 finale. Metcalf made her last appearance in Season 7, episode 13.

Season 8, Episode 20 of "The Big Bang Theory," " The Fortification Implementation," will air on Thursday, April 9 on CBS.

Watch a preview below.