David Underhill

Charismatic and confident – David exudes a natural assurance that impresses and intimidates. He knows he is brilliant, handsome, and talented, and he doesn't need to prove it. His mere presence is enough to crush Leonard .

Manipulative and dishonest – He seduces Penny by making her believe he is single, while being married. He takes naked photos of her and deletes his wife's afterwards, showing total contempt for both women .

Talented and accomplished – David isn't just a brilliant physicist. He's also a skier, musician, biker, and athlete. He's the kind of man who excels at everything he does, which makes Leonard even more jealous .

Direct and opportunistic – He doesn't hesitate to approach Penny as soon as he sees her, without caring about Leonard's feelings. He even offers to take her to the lab to "explore the effects of tequila shots on a gorgeous 22-year-old woman" .

Arrogant but charming – His arrogance is tempered by undeniable charm. He knows how to talk to people, how to put them at ease, and how to get what he wants. That's what makes him dangerous .

The episode "The Bath Item Gift Hypothesis" revolves around:

David's arrival at Caltech – He approaches Leonard in the university cafeteria and asks for his help with his research. Leonard, flattered, enthusiastically accepts and gives him all his phone numbers .

Leonard's jealousy – Sheldon and Howard point out that David has rendered Leonard's work obsolete. Leonard, hypocritically, downplays David's importance, but he is clearly hurt .

Meeting Penny – David meets Penny by chance after Leonard tries (and fails) to ride David's motorcycle. Penny is immediately attracted to him, and David invites her to visit the lab .

Seducing Penny – David takes Penny on motorcycle rides and shows her around the university. Penny, who had always refused Leonard's invitations, enthusiastically accepts. She proclaims herself "Queen of the Nerds" and claims to love science .

Discovering the truth – Penny finds photos of David's wife on his phone. She discovers he is married and that he took naked photos of her before deleting his wife's. She immediately breaks up with him .

Leonard's consolation – Leonard comes to see Penny to talk about their relationship, but ends up comforting her after her breakup with David. He gives her a children's science experiment kit, a nod to her fake passion for science .

1. With Leonard (at the cafeteria)

David: Excuse me, are you Leonard Hofstadter? Leonard: Uh, yeah. David: I'm David Underhill. Dr. Gablehauser said if I wanted to set something up in the photomultiplier lab, that you'd be able to give me a hand. Leonard: You want to work with me? David: Well, if you have a little time, yeah. Leonard: Wow, yeah, sure! Here's my home number, here's my cell, here's my office, here's my parents' number in New Jersey... David: (laughing) Thanks. Congratulations on the MacArthur Grant, by the way. Big fan. Leonard: (to Sheldon and Howard) What are you looking at? You've never seen a hypocrite before?

2. With Penny (after the motorcycle accident)

Penny: So it's your motorcycle? David: Uh-huh. Penny: Oh, is it okay? David: It's fine. Leonard: Yeah, lucky for the bike it landed on my leg. David: You mind giving me a hand with speed racer here? Penny: Yeah, sure. So, um, Dave, how do you know Leonard? David: I'm a physicist. Penny: (laughing) No, you're not. David: Why is that so surprising? Penny: Well, it's just that the physicists I know are indoorsy and pale. David: If you like, I could show you the lab we're working in. We've got some cool toys. Lasers and stuff. Penny: I have always wanted to see a big science lab.

3. With Leonard (after the breakup)

Leonard: Penny, what's going on? Penny: Dave is not smarter than you. He's an idiot. Leonard: Really? Why? Penny: Because a smart guy would have deleted the pictures of his naked wife from his phone BEFORE he took naked pictures of his new girlfriend. Leonard: He tried to take naked pictures of you? Penny: That's all you heard?! He's married, Leonard! Leonard: (silence) ... Oh.

4. With Sheldon (about David)

Sheldon: Leonard, his observation of high-energy positrons has provided the first conclusive evidence for the existence of galactic dark matter. Leonard: So what? Sheldon: So he made all the work you've done useless. Here's two words for you: "big" and "whoosh." Leonard: That's not true! Howard: It is true. Leonard: Fine, maybe a little, but... the guy was just in the right place, at the right time with the right paradigm-shifting reinterpretation of the universe. He got lucky. Raj: And he's very handsome, too.

5. With Penny (at the lab)

David: This is the lab. This is where we work on the photomultiplier tubes. Penny: It's... impressive. David: You know, most people don't understand what we do here. But you, you seem interested in science. Penny: Oh, yes. Absolutely. I'm... the Queen of the Nerds. I'm just nuts for the whole subatomic particle thing. David: (smiling) The last thing I would ever call you is a geek. Penny: (blushing) Well. How about we get a drink?