I'd think it was something just like a Prequel of Girl's last tour.

The submarine captain is obviously meant to be an homage to Vasily Arkhipov, who in real life firmly refused to launch a nuclear missile during the Cuban Missile Crisis, preventing nuclear war.

Of course, sadly, even though they refused to launch the nuke, they still failed, and then it just switched to the story of Yuuri and Chito(´～`)