In the movie, GrandPat (Patrick Star's grandfather) accidentally gets trapped in a vortex that leads him through different dimensions and timelines. SpongeBob, Patrick, and their friends, Sandy Cheeks and Squidward Tentacles, are determined to help GrandPat find his way back home.
Their journey takes them through various worlds with different versions of themselves, including a steampunk version of Bikini Bottom, a prehistoric version of the ocean, and even a dystopian future where Plankton has taken over.
As they travel through these alternate dimensions, they encounter several challenges and obstacles, including facing off against evil versions of themselves. However, with the help of their friends, the group perseveres and finally manages to find a way to bring GrandPat back to their dimension and timeline.
The movie ends with GrandPat grateful to be back home, and the gang reflecting on their adventures and the importance of family and friendship.