It looks like an ordinary sandwich with two giant screws on the upper sides of it.
Role in episode
After Plankton talks about the Mr. Krabs clone he created, he then talks about the Frankenwich and states it is a second grade failure. Karen then corrects Plankton and states that the Frankenwich is actually a first grade failure, with the Frankenwich then squirting out a tomato that crushes Plankton.
It later appears when SpongeBob and Patrick snoop around Plankton's lab, when Patrick grabs the Frankenwich and attempts to take a bite out of it. It then slips out of Patrick's hands and squirts mustard in SpongeBob's face, which causes SpongeBob to accidentally turn on Karen (as she was turned off earlier in the episode).
- The Frankenwich is an obvious parody of Frankenstein.