There really was very little that the trailers didn't already show: they get Chico and Paz on board the helicopter, they find out that Paz has a bomb in here, they take it out, Big Boss disposes of it, and they head back to Mother Base, which is about to be inspected. They get closer to find that it's been under attack. Helicopter lands, Big Boss buys time for Kaz and some soldiers to make it to safety, they take off, Kaz freaks the fuck out about losing Mother Base and proceeds to threaten Paz, who suddenly wakes up and makes it to the door on the helicopter. She notifies the rest of the crew that she had another bomb implanted in her and jumps out, but not quick enough for the explosion to not catch the helicopter, and it goes spinning out of control. The last image you see is another helicopter that it's about to slam into, and then the credits hit.
There's some text that shows up at that point, but it's untranslated. There is a short conversation at the end between Skull Face (I think? Some people said it was Ocelot, but it didn't sound anything like what we've heard of Troy Baker's performance so far) and Paz, who is trying to get information about the whereabouts of CIPHER and Zero. He promises her that Big Boss will be spared if she cooperates, so she agrees to whatever he has in store for her, and the conversation ends right when she's about to give up the location of Zero.