Marshall hears his parents arguing over a missing briefcase. His mother gave it to his dad years ago, and it brings him good luck. He also filled it with some important documents for his big presentation at work, which means it's even worse that he lost it. He claims that he didn't lose it but that it somehow went missing.
To get it back, he decides to go deep into the world of missing objects, eventually locating the central lost and found department. It's basically the place where every missing item in the world eventually ends up. While working to get back the briefcase, he learns that he can make a trade that winds up with him landing in the missing department and his dad getting back his briefcase.
Marshall instead decides to work with the man in the field (who also had a big role in "Gremlins"). He manages to get his dad's briefcase back, just in time for his dad's big presentation. The field worker also takes over the lost and found department, sending the head of the department out into the field. "The Losers" gets high marks for making the department head search the sewers for missing objects LOL.