Jessie, Corey and Selena are best friends and are in a band together. They have big dreams, but all of that comes to a stop when Selena collapses at a rehearsal. Rushed to the hospital, it is discovered that Selena has bacterial meningitis. It's preventable with a vaccine, but since Selena was not vaccinated she now may die. Desperate to save her, Jessie and Corey turn to their mad-scientist neighbor for help. His temperamental time machine might allow them to go back in time and get Selena vaccinated - but it might not bring them back. Jessie volunteers for the mission, and Corey and Dr. Warpton send her back in time. Once she arrives in the past, she has only two hours to find her best friend, convinced her to get vaccinated, and get back in time. But reluctant parents and a glitch in time make Jessie's mission anything but easy.