That was kinda what the first Halo did, you were a badass, cool Space Marine number #43209523 here to kick ass and take names, with ice cold one liners, never flustered in the most woeful of situations.
You're following the story. Humans are fighting aliens, You are loosing. You, humanities last hope, stumble across an ancient relic of unimaginable power, and race against the aliens to harness it's power. With this one weapon you could turn the tide of battle, you rush to find out how to work it, saving all your men, rescuing the commander, stealing directions to the control room, and going there to control it. You reach it, you try to find out how it works...
And then it all goes wrong. You end up on your own in a swamp. Truly alone for the first time ever. You see downed human and alien dropships. The aliens are scared, they are terrified of something. You enter a facility, covered in blood and bodies, you travel further and further inside, the bloodbath of alien forces reaches an unimaginable level, but with no apparent aggressors, no reason for the bloodbaths. There's a soldier firing at you with his pistol, scared into madness by something. You enter a room, the room your soldiers went into...
And then the Flood invade, and it's almost as if they have invaded the game. There's no mention of them in the guidebook, no in game explanation, they look like nothing you've ever seen before, the sheer numbers are mindboggling. The lights around the area are broken, the rooms on fire. The pause menu that always listed your objectives now just says "ESCAPE" In huge capitals.
For those of us who weren't expecting it that was one of the craziest moments in videogame history, truly genre bending, the game went from Duke Nukem to Amnesia in one cutscene.
