Here's the run down of the ending as provided by Michael Akers (HUGE thanks to him!):

Wadsworth reveals that the murder is none other than... Professor Plum. He starts off telling in the same way as the "everybody didit" ending that Professor Plum knew that Mr. Boddy was still alive, etc. But how could he kill the cook? Well! Professor Plum was in league with Mrs. Peacock and they both killed everybody together.

Or did they? The gun is missing, as Professor Plum who has proclaimed his innocence figures out. Whoever has the gun is the murderer.

The murderer is revealed to be... Wadsworth, the butler. He did it all. He goes into a confession about how he always tried to be perfect. He tried to be the perfect husband, but his wife killed herself. He tried to be the perfect butler, but was driven to killing his boss... So, in doing so, he resolved to commit the perfect murder... but that can't be, because there are six witnesses to his confession.

Wrong, he points out. The champagne was poisoned, and all of the suspects will die in a matter of hours unless they get to a hospital. He then runs to the various rooms ripping out the phone cords. He then tells how he is going to lock them in to die... but the doorbell rings. It's the elderly evangelist again.

The evangelist lunges for Wadsworth's gun, and wrestles him to the ground as the other police come in. The police ask whodunit, and wadsworth begins to retell the whole story (as he did at the beginning of the murder revelations).

The cops are watching mesmerized as he tells the story. But when he gets to the part where he opens the door to find colonel mustard, he steps outside, closes, and locks the door. The cops and guests are locked in, but eventually bust out of the conservatory to escape.

Meanwhile, wadsworth has jumped into the car and speeds away... but not totally, because he then hear the "grrrrr" of the german shepard in the backseat.

Fade out. The End.