Sitcom legend Bob Newhart is returning to The Big Bang Theory for the third time.

Newhart will be reprising his role as Professor Proton in an upcoming episode of season 7, TVLine reported Sunday. Professor Proton is the childhood idol of Sheldon (Jim Parsons) and Leonard (Johnny Galecki), a role for which Newhart won an Emmy in 2013.

This is Newhart's third appearance on The Big Bang Theory, after his debut appearance in season 6 and his second in November. The reason for his reappearance as Professor Proton is still unknown, according to the report.

Meanwhile, The Big Bang Theory is gearing up for its big Valentine's Day episode. Set to air on Feb. 6, the episode features Sheldon and Amy (Mayim Bialik) taking "the next baby step" in their relationship in a trip to Napa Valley.

"They have a very real moment and it means a lot to them and to anyone who's a fan," showrunner Steve Molaro told TV Guide recently, adding that he actually teared up on set.

The episode, titled The Locomotive Manipulation, also features Leonard and Penny (Kaley Cuoco) having to rush Raj's (Kunal Nayyar) dog to the vet.

The Locomotive Manipulation will feature Tania Raymonde (Lost) guest starring as Raj's potential love interest. Raymonde will be playing a veterinarian who ends up taking care of Raj's dog.

The Big Bang Theory season 7 airs Thursdays at 8 p.m. on CBS.

Watch a promo for the upcoming Valentine's Day episode of The Big Bang Theory here:

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