Note: Do not take anything about this story seriously, this is not how they'd actually end it and I'd be ashamed and embarrassed if it was.
We are shown the usual shot of the Family Guy house on Spooner Street on a calm afternoon. Peter and his whole family are sitting watching TV. It's a typical cooking where some stereotypical chef is cooking German sausages. Peter's mouth waters in pure anguish over the tasty food. Brian is seen a close-up looking suspicious. He asks Lois if he can take Stewie to the park, she agrees. Stewie shows some resentment but eventually concedes. He follows Brian and Stewie and both walk out the front door. Suddenly a commercial plays advertising a contest with the winner getting an all payed trip with his family to Germany and a tour of Steamy Sausages the world-famous maker of weenies. All he has to do is pick up the phone and make the call but Peter can't since he doesn't know where each of the numbers are on the phone. Lois presses the numbers for him and rings the number where an automated message plays stating that he's been entered. Peter feels accomplished and celebrates by grabbing a beer from the fridge. We cut to a few days with an ecstatic Peter saying he got a letter saying he won the contest. Everyone is celebrating, even Meg who is then pushed to the floor and farted on.
We cut to the family packing their stuff to go to the airport. We get a nice scene of Chris giving the monkey in the closet a heart-warming goodbye and a hug. As we enter the car, Lois explains that Brian and Stewie are on one of their one-off adventures. Peter makes a cutaway gag which involves him being in the multiverse episode during a scene of which he never actually appeared do the joke is pretty confused. We cut to a plane landing in Germany where a bunch of German's are crazy jokes play out never one being funny. We get a cutaway of all the other times the family went to Germany, this would be cool if they didn't randomly put a fake clip in here that only appears in this episode. Peter afterwards looks at the audience says "Yeah we know that last one never happened, we're just messing with ya". We cut to the hotel where Peter immediately jumps in the pool. Then we get a major tone whiplash where Lois gets angry at Peter for embarrassing her, you can hear genuine anger in her voice. Peter doesn't understand why and treats his actions as normal which leads to another cut-away gag.
The story picks up once the family enters their hotel room as Adolf Hitler appears outside the shadows. He speaks in German making Peter laugh while the rest of his family are in shock of the situation they are in. Hitler in rage, grabs a gun from his pocket and shoots Peter first directly in the heart then right through the head. The rest of the family are in utter despair with them all screaming Peter. His blood is pouring out of him, leaving him lying on the floor unable to move and shaking. Peter's no longer laughing, feeling the true fear of the situation. Peter slowing sits up t say one last line, "shut up Meg" then just like that gives out leaving himself lifeless on the floor. One last small mutter of his iconic laugh is heard before complete silence echoes throughout the room. Peter is dead. At this moment, Adolf Hitler grabs a jet pack and crashes through the hotel room giving himself a chance to escape. Lois screams in pain giving her the power to unleash her super form. A green aura swirls around her leaving the two last family members Chris and Meg astonished. Chris makes the funny quip "haha mom is on her period". Lois crashes through the room, reaching increasable speeds trying to catch up to her husbands killer. Meg takes her fathers corpse and throws it out the windows stating "I want a fresh start, our dad sucked" with Chris responding with "hell yeah, I'm going home and throwing a party". I'll save sometime here and just say that Lois fights with Adolf Hitler in sky through whatever attractions Germany has and in the end they both crash into a building both dying in the process.
Meanwhile in the Brian and Stewie subplot, we see them in a random van smoking pot, straight up two minutes of them just smoking pot and laughing at themselves. Brain then drives the car into a burger place. When then randomly cut to Brian with a shotgun and shooting Quagmire in the head thus ending his character arc. Stewie also meets up with Vinnie (that Italian dog that replaced Brian for an episode) who appears with a new owner, they seem happy but it cuts away to something else because no one cares. Brian and Stewie get into a fight about their divorce, insert chicken fight here, insert American dad joke here, insert Cleveland show sucks joke here, insert 12 FOX is owned by Disney jokes here. Long story short, we get one last crazy action sequence in space where Brian sacrifices himself to save the earth (not before one last song sequence) thus ending his life for good. Brian has a funeral where only Stewie shows up, no one else in Brians life cared to show up, same with Peter and Lois.
We then see a time skip to ten years later, Meg and Chris have had two children and Stewie's celebrating his tenth birthday. Chris is serving some cake when he feels a faint touch on his shoulder, it's his father as a ghost telling him "you're the family guy now". Chris gives a big smile as we pan out to the same old house we always see. An orchestrational version of the family guy theme plays as we continue to pan out all the way to a view of the earth but instead of the earth it's Seth McFarlanes face as he looks to the audience and says "what the deuce?"