In August 2018, Plapp won the points race as well as the madison with Blake Quick at the UCI Junior Track World Championships in Aigle, Switzerland. Plapp also placed 2nd to Remco Evenepoel in the time trial at the 2018 UCI Junior Road World Championships in Innsbruck, Austria. Plapp is part of the Cycling Australia High Performance Podium Potential Academy and was selected to compete in the Team pursuit at the 2020 Summer Olympics, where they won the bronze medal.