When Judith comes to drop off Jake she reveals she's going out of town with Herb, leaving the baby with her pill-popping mother. Alan, still concerned the baby is his, jumps at the chance to head over when Judith's mother, Lenore, can't even turn the alarm. Meanwhile, when he gets there, Lenore is popping muscle relaxers and drinking wine like a fish, while he bonds with his daughter. Soon, a drunk Lenore confesses she hasn't been touched in 14 years and attempts to seduce Alan. Meanwhile, Judith, unable to reach Lenore, calls Alan on his cell phone and asks him to head over to make sure everything is okay. He, of course, pretends like he isn't already there and agrees. Alan ends up losing Lenore when they go out for a drive and she takes off out of the car.
Jake, who recently got his learners permit, has been harassing Charlie to drive his $80,000 Mercedes, who is, of course, unwilling. Charlie agrees to drive him to get some food, and while reversing, he thinks he ran over Chelsea's cat, Sir Lancelot, who ran away earlier. Jake takes a picture of the roadkill before Charlie can clean it up and uses the pic to blackmail Charlie into letting him drive the car. When Chelsea comes home later she finds Sir Lancelot outside and Charlie has no idea what cat he ran over, but he's out of Jake's grip. That is until a very large neighbor comes over looking for his cat.
The title, "Mmm, Fish. Yum." was said by Alan as he went to go pick up some salmon shortly after Chelsea and Charlie discussed how cats think.