Twilight has found the location of a rare manuscript said to belong to a famous magical artisan. Only thing is that it's located at the bottom of Mermare's Reach, one of the deepest sea caves in Eastern Equestria. Thanks to a magical charm, such a cave can only be traversed without diving equipment, as any equipment will be vaporised upon entry to these enchanted waters. Twilight promises to return with the scroll, with Spike accompanying her in case things go wrong.

It's a race against their ailing lungs, will they be up to the task?