When Oscar isn't invited to Alexa's birthday party, Mortimer tries to make him feel better by telling him about a similar time in his life. Ever the glowworm, Oscar sneaks back in time to help a young Mortimer overcome the sad experience.

Mortimer and the monsters must figure out how to send Oscar back to his own time, but the only person that can help is the foe of monsters everywhere: a witch. Will they all be able to work together to get Oscar back to Hedgehog?