Mystery Meat

Mystery Meat is a quest in Fallout 4.

Detailed walkthrough
Upon arrival at Longneck Lukowski's Cannery Theodore and Rylee will be arguing about her ability to sell his product and mention people getting sick. Asking him about it after she leaves will give you the option to offer to help him out by clearing the mole rats from the building. He'll give 50 caps up front.

Despite him mentioning that you shouldn't go into the basement there's nothing in the cannery itself to do, except get the Barter bobblehead and the Tales of a Junktown Jerky Vendor magazine right next to it, so head up the elevator in the SE corner and go down, on reaching the bottom the elevator will be locked out by Theodore. Read the terminal to get a little background, then push on. The room straight ahead is trapped and has some minor loot and critters, there's a good chance the radroaches will trip the trap and die on their own.

A bit further along is a room with some ghouls and a hole in the wall, continue through, killing them as you go. The tunnel is fairly linear, but at the first place where you're required to dip into the irradiated water there's a area that can be reached BEHIND the stairs with a few extra bits of minor loot.

After a while you'll get back to the factory and find out that the meat Theodore's been selling has some ghoul in it as well, at that point he'll be hostile and you'll have to kill him, finishing the mission. Alternatively, if his health becomes low enough during the gunfight, he will plead with you. He offers to run the business, and split the profits with you. You can either agree or kill him, finishing the mission.

Bugs
Talking to Trader Rylee before Theodore will cause a dialogue bug that removes the option to ask him about people getting sick, you can still start the quest by going all the way up the inside of the cannery and going down the elevator in the SE corner.