Complex sentence: Santa's Little Helper goes on to replace Duffman as the mascot for Duff Beer, but the Simpsons soon miss him terribly and enlist the help of Duffman to get him back.
Split into simpler sentences: Santa's Little Helper goes on to replace Duffman as the mascot for Duff Beer. But the Simpsons soon miss him terribly and enlist the help of Duffman to get him back.