A perfectionist contract killer is drafted by two mobsters to assassinate their Don. Twenty nine hours later in a bizarre twist of fate his meticulously calculated hit goes wrong and he kills the wrong man. He has only two hours to learn the identity of his betrayer.

JanCabal's example is pretty close, I think it just needs your polishing touches on it :)

Definitely don't need the twenty nine hours later, we almost don't even know who the original hit was for, we just need to know (as far as the setup goes) that "When a perfectionist hitman takes out the wrong target ..."

And we need to know the stakes. I second the motion that Jan puts forward - if he fails, the kitty gets it.

The best, how to start a logline, is with After, As, While, When, as it automatically gives you a tool to tell as as many information in one sentence as possible.

After his assassination goes wrong by mistakenly removing wrong character, perfectionist killer has only two hours to identify his betrayer or ? what's at stake? Atomic bomb explodes/world crashes/kitty dies.

Hopefully it helped ;)