It simply looks like the regular Krusty Krab, save for a few minor additions. At the front, it is painted deep green and there is a picture of Plankton's eye and his two antennae at the top. The sign also read's the restaurant's name on the Krusty Krab's clam sign.
The words "the" and "Krusty" are written in the same font as the Krusty Krab sign, but the "Krab" part is covered by a white wooden plank that reads Plankton's name in dark green text. The interior looks exactly like the one for the regular Krusty Krab.
Role in episode
Plankton lies to his grandmother that he owns the Krusty Krab. He then asks Mr. Krabs to temporarily hand ownership of the establishment when his grandma visits. Mr. Krabs is slightly reluctant at first but eventually agrees. Therefore, they make a few adjustments to the restaurant since Plankton temporarily owns it.