Some reviewers seem to have taken this film way, way too seriously!
I'm not going to re-describe the plot as others have done so already. I just want to share my opinion (& try to counteract some of the, imho uneccessary, vitriol in the 1-star reviews)
It's a hoot!
Subverting the ubiquitous "grays are good" theme that pervades virtually all SF in which they feature is an brilliant starting point.
The animals (again, imho) are not "running away"; they're essentially 5th columnists, spreading the 'infection' as they scatter.
Adding what is, in effect, an industrial strength 'bottom joke' theme may seem puerile but serves to point up that the 4 (5 including Duddits) main characters are still boys at heart simultaneously hearkening back to when they all met; as young boys.
It is *not* anywhere near as good as the book but which film ever is (or ever has been or ever will be)?
One last thing. Damian Lewis' accent as Mr Gray is superb (judging by the surprised/scornful comments by some of the other reviewers, I should also remind you that Mr Lewis is, in fact, English) and serves to accent (sorry) the changeover between the original character & Mr Gray. Coupled with a truly excellent maniacal grin, I found this bit of distinct characterisation really added to the joy of this film.
So, as one other reviewer has already suggested, give it a try. Suspend your disbelief and enjoy the fun!