Daffy Duck owns a "Mouseburger Cafe" in the middle of the desert. Because of his tight budget, he uses rubber mice rather than real mice for his burgers (perhaps an unintentional foreshadowing of where the fast food industry would be headed 30 years later). But when a Mexican bandito cat named El Supremo discovers this, he demands that Daffy brings him real mouseburger. Unfortunately for Daffy, the only mouse in the area is Speedy Gonzales.
Daffy/Speedy cartoons are typically very hit or miss. There's a mix of good and bad cartoons in the series. And fortunately, this cartoon falls under the "good" category. Daffy, for once, is actually a fairly sympathetic character with an understandable reason for going after Speedy (so that he can avoid being massacred by El Supremo - who, by the way, is a funny character in and of himself). He's not really a "good guy", per see. But he's considerably less evil and more sympathy-inducing than in other D/S shorts. There are plenty of solid gags, such as the garbageman mistaking Daffy hiding in a trashcan for "talking garbage." And while the ending's a bit of a downer, since Daffy winds up the loser when it really should've been El Supremo, it doesn't spoil an otherwise solid cartoon.
Overall, this cartoon stands with Daffy Rents and Snow Excuse as probably the three best cartoons in the Daffy/Speedy series. That, of course, isn't saying much. But when taken by its own merits, Daffy's Diner is a nice little cartoon. And a fine farewell to the DFE-era of WB cartoons.