The Swooner and the Thrush originally fought over their radio spots, but are now in love. This gives Clark the idea to hold their wedding in the studio as a publicity stunt, and have them sing duets during it.
The Thrush's publicity agent Marvin Maxwell objects, a crazed female fan objects, The Thrush's crazy uncle comes out of the woodwork and objects, but it goes on anyhoo. The couple say their vows on air, and sing "Oh, Promise Me," and everything goes great right up until the moment they get in the limo and eat poisoned sandwiches.
Midnight figures out it was Marvin Maxwell, who is also The Thrush's cousin, and who would have inherited crazy uncle's wealth if he killed both the Thrush and the uncle, which he was getting ready to do, before Midnight showed up and caught him.
Then it turns out the Thrush survived being posioned, but not the Swooner.