Arthur gets kidnapped by a group of thugs and is going to be used as a hostage so the group can use their getaway car. But before they can drive too far off Martin takes a pistol and hits one/some of the baddies. The rest get caught and Arthur comes out fine, if shaken. Everyone's surprised thatartin can actually handle a gun without shooting his foot off, but whether he offers an explanation is up to anon.
And if there's some influence from one Jim Moriarty in there, well...that's bonus.