Welcome to Mars City. You are a new Marine trasnfer here on Mars. After making your way through 'the dungeon' (which was Mars City Underground), you find yourself back at the Lobby, but, you're a Marine, so Sargent Kelly called you to his office. And you were running late because you had gotten hooked to Super Turbo Turkey Puncher 3

After you make it to Marine HQ, Sargent Kelly was waiting for you. When he noticed you, he crosses his arms, before giving you a assignment

"Took your sweet time, Marine? Now, here's the situation. A member of the Science Team has gone missing. Since you're the ranking FNG, you get to find him. I programmed a Senrty Bot to guide you to the elevator there. Hope you follow the Sentry better than you've followed orders so far. Move out."