Riley finds an old song that her Mom wrote long ago and Megablast decides to sample it in their latest song. Meanwhile, Dr. Dandelion has created a giant Jack and the Beanstalk-esque plant penthouse over Greenville. When the beanstalk's vines start swiping things to furnish the new pad, Megablast must transform into Rocket, Thunder and Metallix to stop the pilfering plant before the weight of all the stuff it's swiped makes it collapse on the town.