I would recommend Frontline Plus too, it works very well at my house.
My Vet stopped selling Advantage that I used to use, but this stuff is even
better & the cats don't run because of the smell. I can get close to them
with an open dose & put it on them easily. (added points in my book)

If you already have fleas inside, you'll have to clean their bedding &
everything else inside. It's a real pain, but a 2 front approach works best.