Team Scorpion has a new handler, Deputy Homeland Director Catherine Cooper. She has a classified mission for Cabe, Happy, and Walter. They board a top secret Navy submarine while Toby, Sylvester, and Paige are back at the garage.
Walter realizes that their mission is going too easy, and sure enough, it was. There's an explosion, and now the sub has crashed to the ocean floor.
Walter and Happy make a suit that gets Cabe to the surface. Cabe gives Sylvester and Toby Happy's list of ingredients to make an oxygen tank, and they are going to need help from Happy's dad.
Back on the sub, Walter figures out a way to free most of the crew, but it means that his, Happy's, and their navy friend's rescue will be more difficult. Basically, they change the air pressure in their cabin, so they can't just swim out of the sub. They need a sub to rescue them.
The team manages to send an oxygen tank down to the sub, but it doesn't matter. The sailor initiates the self destruct sequence. Walter and Happy need to come up with an escape plan in four and a half minutes.
Don't worry, the team makes it out alive. There's even a dance party to celebrate.