Something Fishy This Way Comes is episode 5 in Season 2 of Disney Channel Original American Dragon: Jake Long.
Jake's life is thrown into chaos when Grandpa starts dating his principal, Ms. Decerto. Meanwhile, Jake learns that a ferocious, shape-shifting beast is preying on magical creatures, and suspects that his school principal is the creature. Jake learns that the principal isn't and is in fact a mermaid detective after the two suspect each other of being the creature. In the end, Jake, Decerto, Trixie and Spud defeat the creature and Jake accepts his grandfather's new romantic relationship, but with the creature captured, she has to return to her underwater home.