Married But Available is a fashioned Hong Kong slang to describe the hidden marriage couples which is initially known as MBA.
In order to get the high salary job, Chu Ming-ming decides to hide her marriage from her boss.
Coincidentally around the same time, Tony is also hired as a personal assistant by his best friend, Lucas Cheung, whom he met while doing his master's degree in America.
Through his job Tony becomes reacquainted with Kay, Lucas' younger half-sister, who he met years earlier when she was a teenager.
Tony had promised Kay that once she learns how to swim he will teach her to dive and show her the colourful world under the sea and find the beautiful mermaid.