It seems somewhat surprising, considering how obvious the idea now seems, that Man-Bat didn't make his first appearance until Detective Comics #400 (June 1970). Had the character appeared prior to Denny O'Neil's and Neal Adams' work on Batman then he would probably never have been as horrifying the character that Frank Robbins created.

Kirk Langstrom, a scientist specialising in bats working for the Gotham Museum of Natural History, was secretly working on a method of giving humans the kind of sensory information about their environment that bats receive. He followed the great, and dodgy, science fiction tradition of developing a serum to give the person who takes the serum some of the powers of the bat. So, when he took the serum himself it had the frankly predictable consequences.

Man-Bat: "It was your inspiration...your great fight against criminals of the night -- that brought this on me!"
Batman: "Then if I was your hero...Why hide from me--?"
Man-Bat: "All right...SEE!!"
Batman: "Man--! You're even more of a 'bat' than I ever was! And with your added ability to 'see' in the dark -- you've topped me! What an awe inspiring disguise!"
Man-Bat: "Skreek... Disguise? DISGUISE...? I only wish it were!" Detective Comics #400

Batman was certainly impressed by this new Bat character, especially by what he thought was an incredible costume. It wasn't until their second meeting in Detective #402 that Batman realised that there really was a man-sized bat in Gotham. Kirk Langstrom initially wanted to be a hero like Batman, but didn't count on becoming a bat. The relationship between the Batman and Man-Bat was always going to be confrontational; Kirk Langstrom's human and the bat sides are in confrontation. Kirk Langstrom's desire was to be with his fiancee Francine, while his bat instincts were trying to take over and have him live a normal bat lifestyle also requiring a mate.

As is traditional with Batman he just wanted to help Man-Bat to lead a normal human life (see virtually any Two-Face story) so he developed an antidote to the bat serum. Unfortunately, the bat in Man-Bat did not intend to give up his life and his power convince Francine to join him as a She-Bat. Batman resolved the original Man-Bat tale in Detective Comics #407 by injecting both Kirk and Francine with his antidote but this is comics so they were never destined to have a nice quiet life together.

Man-Bat appeared in solo stories in the Batman Family comic where he helped Jason bard solve a few cases and has continued to appear occasionally ever since, most notably in Batman: Man-Bat a 3 issue prestige format Elseworlds series and his own 3 issue series.

Kirk and Francine are married and now have two children Rebecca and Aaron. Unlike all of the others Aaron was born a little Bat-Boy.