While exploring, Nobita discovers a rocket and frees a tiny alien named Papi, who is being pursued by the PCIA army on his planet. Doraemon and his friends struggle to adapt to Papi's small size, but they use a "Small Light" gadget to enhance their playtime. However, Papi's enemies come to Earth and attack Doraemon and Nobita. Feeling guilty, Papi bravely fights back. To safeguard Papi and Pirika, Doraemon and the group embark on a journey to the alien planet.