Well, apparently without a lack of opposite-sex intellectual equals, mermaids have essentially become bisexual. They mate with the males for procreation, but often form romantic relationships with their "sisters." Family bonds are not strong; mothers are called "matria" but most young, male or female, are raised in a communal creche.
The society lives in magically-shielded cities under the ocean. In the past, this has been fine, but as humanity's technology improves, it's getting harder to keep them away. The breaking point is coming; eventually they won't be able to hide anymore. There are cities worldwide, but it looks like the one off the American eastern coast is going to be the first to break the silence. When a pod of males is captured and taken aboard a commercial research vessel, the MC is sent to free them.
She does - spectacularly. So the silence is broken.
She and another mer will be sent to the mainland to try and broker peace, or else. The queen is pissed off, and not really interested in peace, only retribution, so time is limited.
And that's all I've got. I do know that negotiations will be unsuccessful, and war will result. After that... who knows? Maybe the merfolk will take over the US. ;) Maybe there'll be a rebel alliance.
Oh, and I have a Senator from somewhere named Mason. And the MC's best friend/sometimes lover is Aria, the land-spy.