It's been 3 months since Viper and the other Slytherins caught Severus singing, and the Incident was never spoken of again. As for Dumbledouche's plan, he was going to have Ron and Hermione steal  Severus's selkie coat and lock it into a trunk that he'll throw into the Black lake. He knows that without their coat  a Selkie has no voice.

Severus's P.O.V:

I'm now four months pregnant with twins, it's January now, and Poppy said that I'm due in June. 5 more months until I meet my babies.

Suddenly,  I feel a pain in my throat. I tried to speak but no words came out. I checked my room and something was missing. That's when it hit me!  My coat! Someone took my selkie coat!

I thought that it would be fine without my coat, but I was wrong.

Third Person P.O.V:

It's been about 4 months since Severus's selkie coat was stolen, and he hasn't spoken a word. It is now May 16, but the Slytherins and Gryffindors in his 4th year Potions class realize the consequences of separating a selkie from its coat.

Severus was writing the instructions for the Pepper-Up Potion they were going to do, when he suddenly collapsed and started coughing. Then, two strands of his hair turn white.

Viper immediately rushed over to her mom and helped him into a chair. "Are you okay, Professor?" she asked worriedly, earning a shake of the head in return. Then, her eyes widened in horror. "Your coat is gone?" Viper asked her mom, earning a nod.

Viper turned to the students,  her icy glare firmly in place, making the Gryffindors shudder in fear. "Class is dismissed. Slytherin stay behind. Mom's coat was stolen and without their coat, the Selkie has no voice. Without his song, he won't last the next full moon. It's almost the current full moon, so a week and a month."

All of the Slytherins gasped in shock. Their Head of House and surrogate mother was dying.

Now, they needed to find the coat before it's too late.