Actor and nerd idol Nathan Fillion will make a special appearance on "The Big Bang Theory," while the show will pay tribute to late actress Carol Ann Susi in the next all-new episode on Feb. 19.

In last week's episode, "The Troll Manifestation," Leonard and Sheldon published an essay online explaining a new scientific breakthrough on super fluids. Although the thesis received positive feedback from many scientists, they soon became subjected to cyber harassment as an Internet troll ruthlessly criticized their work.

Refusing to back down, Sheldon went toe-to-toe with the troll and even requested to a face-to-face video chat. That's when they discovered that the troll was actually Stephen Hawking. It turns out that the professor was just messing around with Sheldon and he really found the paper to be intriguing, reports Entertainment Weekly.

In the upcoming episode, titled "The Comic Book Store Regeneration," "Firefly" and "Castle" star Nathan Fillion will make a cameo on the show playing himself, reports EW. During the episode, the geeky gang will be in utter disbelief when they meet their idol in a restaurant. Leonard and Raj will also debate amongst themselves if it is really him or not and whether or not they should ask him for a picture.

The episode will also honor Susi who played the voice role of Howard Wolowitz's nagging mother. The 62-year-old actress died last November.

According to TVLine, Howard will struggle to cope with shocking news concerning his mother. "The (presumably) tragic turn of events coincides with the reopening of Stuart's comic book store -- a shop that, much to Howard's shock and awe, features a new seating area filled with furniture from his mom's home."

The CBS comedy also released a preview photo that shows Howard in a state of sadness while he seems to be sitting inside of Stuart's new comic book store, notes Carter Matt.

TV.com reports that the synopsis reads as follows: "Amid the excitement of Stuart reopening his comic-book store, Howard gets some unsettling news; and after teaching Sheldon how to 'let it go,' Penny doesn't practice what she preaches when she learns something maddening about Amy. Meanwhile, Raj and Leonard run into a restaurant customer who looks a lot like actor Nathan Fillion."

Season 8, Episode 14 of "The Big Bang Theory," "The Comic Book Store Regeneration," will air on Thursday, Feb. 19 on CBS.

Watch a preview below.

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