The crew of the recommmisioned Bolao-class submarine USS Stingray are taking part in a war game to test the preparedness of the United States Navy. Imagine their surprise when they randomly find themselves in Ponyville. Imagine their shock when they realise they're now ponies.
The girls of Ponyville were having a perfectly normal day, then they start meeting ponies who claim they're actually some kind of ape-creature. Clearly, someone is up to shenanigans.
A crossover of My Little Pony with Down Periscope