The Big Bang Theory is ramping up the drama leading up to its milestone 200th episode, andThursday's Valentine's Day episode has a big surprise in store.

Everybody is dealing with Cupid's holiday - or as Penny (Kaley Cuoco) calls it, National Sex Night — in their own way. Sheldon (Jim Parsons) and Amy (Mayim Bialik) use Sheldon's first live episode of Fun With Flags as a way of masking their real feelings for each other, while Raj (Kunal Nayyar) has an existential crisis. Leonard (Johnny Galecki) and Penny also add some spice in their marriage after some unexpected fun during a night out.

But it's Howard (Simon Helberg) and Bernadette's (Melissa Rauch) Valentine's Day that'll prove memorable to them for years to come, after Bernadette reveals a shocking secret.

The Big Bang Theory airs Thursdays at 8/7c on CBS. Check out a sneak peek of the episode below.

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