The Big Bang Theory season 10 returns with its pre-finale episode this Thursday (4 May), which promises a big mystery as Sheldon Cooper, Leonard Hofstadter and Howard Wolowitz complete their top-secret air force project.

TBBT episode 23 is titled, The Gyroscopic Collapse, which airs on 4 May at 8pm EST on CBS Network. The official synopsis reads as follows:

After Leonard, Sheldon and Wolowitz celebrate the completion of the top secret air force project, they are met with an unpleasant surprise. Also, Amy is offered a summer position as a visiting researcher.

A promo for the episode shows the gang celebrating the successful completion of their air force Gyroscopic project. They all cheer with champagne in their hands as Sheldon points out, "No bendy straw. Some party!" Bernadette wants to know what happens next and Howard replies, "We see the movie based on our lives, starring more attractive versions of us."

However, there is a twist, as the promo voiceover warns, "Don't miss the big mystery." We then see Sheldon, Leonard, and Howard entering their office only to find it empty." Leonard asks, "Where is everything?"

Sheldon says, "This is very disconcerting," to which Howard quips, "But the movie did just get good."

Later we see Sheldon and Leonard entering their apartment only to find it empty too. Sheldon yells, "They are erasing our lives."

Besides, Amy Farrah Fowler has some big news to share with her boyfriend. She tells Sheldon, "I was offered a fellowship at Princeton." The physicist quips, "Where Leonard got his Ph.D? So it may have gone downhill."

Actor Dean Norris returns as Colonel Williams in the episode. Click here to watch the premiere episode via live stream online. You can also click here to watch it online (link available only in the US).