When Stephanie arrives in LazyTown, to spend the summer with her Uncle Milford, she finds that she has trouble making friends because all the local kids spend all their time indoors playing video games. So, her uncle enlists the help of Sportacus, an athletic hero who encourages the kids to have fun by remaining active. Unfortunately, this doesn't sit well with Robbie Rotten, a lazy cheat, who takes pride in having LazyTown be the laziest town around, so he plots to get rid of Sportacus.

The kids must face Robbie Rotten in a sports competition without the help of Sportacus after the villain causes the athletic hero to suffer a Sugar Meltdown brought on by a candy apple.

Ziggy tries to be a hero just like Sportacus, but his antics start getting him in trouble when he messes up the construction of the kids' new clubhouse.

Sportacus tries on a new pair of shoes, unaware that they are actually radio-controlled boots that Robbie will use to ruin Sportacus' athletic skills during Spectacular Sports Day.

Robbie causes panic in LazyTown when he disguises himself as Dr. Rottenstein and makes everyone believe that vegetables are at the root of a new disease.

Robbie tricks the kids into going on a treasure hunt for the LazyTown Stone and then tries to steal it from them until Sportacus arrives to stop Robbie, much like a mysterious hero once stopped the famed Rottenbeard.