Plan Z-99 (pronounced Plan Zed dash niner niner) is a plan mentioned in the episode "SpongeBob's Last Stand." The plan was made by Mr. Krabs and is about him being forced to sell the Krabby Patty secret formula to Plankton. This plan would be only used as a last resort, which would be when the Krusty Krab would undoubtedly go out of business, then Plankton will take over it and the rest of Bikini Bottom.
Plan Z-99 is almost executed in "SpongeBob's Last Stand." Before Mr. Krabs can sell the formula to Plankton, the Chum Bucket is attacked by jellyfish. This is the only recorded time Plan Z-99 had to be used.