This 1954 movie is the technicolour sequel to the black-and-white film Miranda, and features the return of the title character of that film. Miranda, the cheeky and free-spirited mermaid who's fascinated with love and men is back to her old tricks, but with a few changes.
This sequel adds some backstory for Miranda, namely that she's related to humans. Miranda's a distant cousin and the spitting image of Caroline Trewella, a schoolteacher who's on holiday in Cornwall and encounters Miranda by chance. Miranda is bored because the mermen are away, so she asks if she can switch places with Caroline for a week to get to know the handsome men in town. Caroline agrees, and secretly goes off on cycling trip with a friend while Miranda takes her place. Miranda also takes it upon herself to find a new beau for Caroline, since she's thoroughly unimpressed with Caroline's stuffy fiancé.
It's similar to the first film but with the bonus of new misunderstandings due the switcheroo, plus Miranda's flirtations are also for the benefit for her dear cousin whom she hopes will have a happily ever after. The climax is quite exciting, in that one of Miranda's new acquaintances has figured out that she's a mermaid and tries to out her at a public concert, only for that to backfire when Caroline comes back and the two work together to sabotage those efforts.
This film also tries to up its mermaid game by featuring some swimming and underwater scenes (trying to compete with the earlier Mr. Peabody and the Mermaid, maybe?) but the technology is still very rudimentary because Glynis Johns is visibly struggling to act while swimming at the same time.