Isn't he supposed to get the formula back? He doesn't know Knox and with just a few words from
Daphne he decided to join them?

Well, who has the formula? Daphne! Well, ok, her boss! But basically, joining them bring him a lot closer to the formula than he is right now! ''Be close to your friend and closer to your enemy'', I think Hiro starts to take this expression a little bit too seriously! lol But that's the plan! Well, I think it is, but we'll get confirmation in a future episode, I guess!

Also, between daphne's line "my friend here says you're looking for work"

And Knox asking on the phone what to do about the "japanese guys" after kidnapping adam, it looks like there was a deleted scene in which knox approaches hiro and ando, and seeing that hiro's always saying things like "I'm here to save the world!" Knox said that they weren't looking for heroes and sent the pair back to the bar.

For the sake of the plot.

I think this is one thing you can't say is for the sake of the plot. He wants to be a spy, he wants to find out who has the formula and get it back - only problem is he was no idea who he's dealing with and that they knew his father - Hiro better watch out he's being played as much as he's playing them. It makes sense that Hiro would think going in would be the only way.

It's not just for the sake of the plot. It's typical role-players' thinking. We must meet the big baddie! - Well, we'll just hang out in his recruitment cantina of choice then! My half-elven ranger is currently doing just the same thing. Doesn't Hiro strike you as the type of guy who appreciates a good round o' D&D?

Daphne stole the formula right? So she would be the best person to get close to if you wanted to get it back. So workng with her to get info on the formula is the best thing he can do at the moment.

Haha thanks guys
Anyway Ando asks the same question in ep 6...

To get to the big bad, pure and simple.