Mr. Plankton tells the story of Hae Jo (Woo Do-hwan), a man with a terminal illness, who seeks closure by finding his biological father. His path crosses with his ex, Jo Jae-mi (Lee Yoo-mi), and they embark on a road trip, where they confront past regrets and their complicated feelings for each other. Jae-mi’s infertility and relationship struggles with her fiancé, Eo Heung (Oh Jung-se), add further emotional depth. In the series’ poignant finale, Hae Jo accepts his fate, while Jae-mi faces the difficult choice between her past and future. The ending offers a bittersweet but hopeful message about love, loss, and personal growth.