The Big Bang Theory season 9 episode
Sheldon and Amy in The Big Bang Theory season 9 episode 15CBS

Sheldon and Amy will spend their Valentines Day by doing a live episode of Fun With Flags in the upcoming chapter of The Big Bang Theory season 9. Episode 15 is titled The Valentino Submergence, which airs on 11 February at 8pm EST on the CBS Network.

The official synopsis reads as follows:

Sheldon and Amy celebrate Valentine's Day with a live episode of "Fun With Flags"; Leonard and Penny feel the pangs of aging; Howard and Bernadette are surprised by what they find in their hot tub; and Raj can't decide between Emily and the comely woman he met at the Comic Bookstore.

Click here to watch the episode via live stream online on CBS All Access. You can also click here to watch it online (only in the US).

The upcoming episode of season 9 will reveal a big secret. In the latest promo, the voice-over declares, "You saw The Big Bang wedding, the breakup, the hookup, now the show returns with the biggest Big Bang secret ever!"

Warning: Spoilers ahead

According to a leaked report, the big secret the promo teases is related to Howard and Bernadette's little bundle of love. Bernadette will tell Howard that she is pregnant by the end of the episode.

The episode will also find Leonard and Penny struggling to grapple with the fact they have become an old married couple. Another preview for the episode shows, Penny and Leonard acting young as they yell, "Happy Valentines Day...We are fun and young."

Then scene then shifts to them cleaning up as Penny says, "Cleaning up is not young and fun!"

To know what is the big secret, tune in to the Big Bang Theory that airs on 11 February on the CBS network.