A pitcher named Kelly is added to the St. Louis rotation and he plays a big role in their making the post-season.
That is basically what happens in the movie "It Happens Every Spring" starring Ray Milland.
In the video below, go to about 34:30 and then watch for about 20 seconds. You will see what a great pitcher Kelly (Milland) was and why his team won the pennant. Click here to go to a page where you can watch the whole movie