Dr. David Underhill is a Macarthur Grant recipient and a very successful physicist , with Leonard excited to be working with him. His observation of high energy positrons has provided the first conclusive evidence for the existence of galactic dark matter and Dr. Gablehauser said if David wanted to set something up in the photomultiplier lab, Leonard would be able to give him a hand.
After Sheldon and Howard assert that Dr. Underhill made all the work Leonard has done useless, Leonard hypocritically says "the guy was just in the right place, at the right time with the right paradigm-shifting reinterpretation of the universe. He got lucky." Once David approaches Leonard in the cafeteria about their work examining the radiation levels of photomultiplier tubes for a new dark matter detector, Leonard gives him several phone numbers to reach him and states he is a big fan.
Ecstatic to go to the gym with him, Leonard reveals David is a Black Diamond skier, collects vintage motorcycles, plays in a rock band, and is good at impersonating Stephen Hawking's computer generated voice like Howard.
Meanwhile, with his leather jacket, motorcycle, and handsome looks, Penny is shocked (as most of the physicists she knows are indoorsy and pale) to know that David is a scientist and starts dating him, prompting her to state that she is into science, is the "Queen of the Nerds", and that she always wanted to tour the university - facts that Leonard contradicts because he knows that she is not into science, as she has never expressed interest, and she always turned down his offers to provide a tour of the university for the last year and a half, reasoning she had yoga to attend.
They break up as she is dismayed to find out he is married. He took some naked pictures of Penny before he removed the naked pictures of his wife. When she tells Leonard, at first, all he hears is that David was taking naked pictures of Penny. Leonard is confused as to why Penny thinks she is not smart enough to date him but can date David, and cries, "David Underhill is ten times smarter than me! You'd have to drive a railroad spike into his brain for me to beat him at checkers! Next to him, I’m like one of those sign-language gorillas who knows how to ask for grapes!" To which Penny retorts, "Dave is not smarter than you. He’s an idiot." He tried to take some naked pictures of Penny and accidentally shows her the same think of his wife. End of story.
Interestingly enough, Michael Trucco also plays Samuel T. Anders on Battlestar Galactica, of which the guys are avid fans.