The first episode was the origin. Everybody (at least everyone who has the box sets) knows that.
Basically, Mr. Peabody sees a bunch of nasty kids picking on Sherman, and he feels so sorry he tries to adopt him (much like he was getting a puppy out of a shelter, and they exploit the reversed roles angle for the three minutes) he then appears in front of court to try to adopt him, and Peabody builds a time machine because Sherman, like a puppy, was messing up his penthouse apartment. Then of course, they wind up getting electrocuted by Benjamin Franklin, causing Mr. Peabody to invent a "What If" machine so they don't screw up history, and wind up getting electrocuted by Benjamin Franklin.
Now, I gotta admit, Bullwinkle cartoons make uneven movies. George of the Jungle and its sequel were very good, and very in the spirit despite changing a few things (the sequel even made fun of the fact they couldn't get Brendan Frasier). Dudley Do-Right was bland, poorly cast, and only the guy playing Snidely Whiplash got the point. I liked it when I watched it first, but the second time just felt bad. It even took place in the WRONG time period. Sure, there was a LOT of anachronism in the original program, but it was ALWAYS at the turn of the 20th century. Rocky and Bullwinkle I actually liked, but the fact they threw a useless Mary Sue in there wasn't a smart idea and ruined what they were doing.
And by all counts, they were all wildly unsuccessful. They've been threatening to make this film for years, and I still doubt they're going to fully go through with it.