James invites Arthur and D.W. to swim with him, but Arthur is shocked to find out that Molly is James' older sister. Arthur and Molly have fun together and become friends, but avoid each other in public because their own friend groups will not approve. Grandma Thora finds a stray dog named Killer on her porch. Killer turns out to be a bully, intimidating D.W., Pal, and Amigo. Kate and the other dogs doubt that they like Killer, but reconsider after Killer rescues Nemo from a tree.