Lord and Lady Clambakersfield have returned to Atlantica to have their toddler mertwins blessed by Triton. Ariel volunteers to babysit the twins while her father gives the babies' parents a tour of his kingdom. The babysitting gets off to a shaky start when the twins get into everything and are kidnapped by the Lobster Mobster. When the twins accidently get stuck in a bubble from the Lobster Mobster's bubble gun and are almost eaten by a giant squid, it's up to Ariel & friends to rescue them. In the end Ariel concludes that despite everything babysitting is pretty fun and volunteers to do it again whenever the Clambakersfields come back to visit.
(It's helpful that Ariel loves babies, considering the timeline of that episode and when she had her own daughter Melody in TLM2, which was supposed to be set only 2-3 yrs later.)