Steve was stuck for a moment, staring at the woman. She... she was taking him along? But he had nearly passed out only after running two blocks! When she started moving away, Steve pulled himself upright, forced himself to even out his breathing, and followed her. If she really was going to let him come along, he was going to prove that he could.
Once they were in the cab (which Steve was very, very grateful for), he turned to her. "So, Doc, what's a TARDIS? And... how does the head of the FBI owe you a favor?"
She was obviously British. Maybe it had something to do with the war that was raging overseas?