Mentally unstable scientist and struggling aspiring inventor Wayne Szalinski (Rick Moranis) is working on a revolutionary laser gun that can miniaturize objects. One morning after a failed test, Wayne goes off to a conference and leaves his responsible daughter Amy (Amy O'Neill) in charge. However, their next door neighbours Russ Thompson, Jr. (Thomas Wilson Brown) and his brother Ron (Jared Rushton) turn up at their doorstep to retrieve Ron's baseball from the attic where Wayne works after Ron accidentally broke the glass window playing a game of Baseball. But, Ron's baseball accidentally causes the laser gun to activate and Amy, Russ, Ron and Amy's dorky little brother Nick (Robert Oliveri) get zapped and are shrunk only to find themselves in the lawn of their backyard when Wayne comes home and angrily smashes up the laser gun and sweeps up Amy, Nick, Russ and Ron and puts them in a black plastic rubbish bag. Amy, Nick, Russ and Ron embark on a dangerous journey through the jungle-like lawn as they set out to get home. But, Wayne soon learns that the laser gun works and realizes it shrank Amy, Nick, Russ and Ron and sets out to find them and repair the laser gun and restore them to normal size before his estranged wife Diane (Marcia Strassman) arrives and Russ and Ron's cantankerous father Russell, Sr. (Matt Frewer) and their mother Mae (Kristine Sutherland) leave to go on a fishing trip and soon becomes concerned and report Russ and Ron to the police as missing.
