The film begins with Nobita waking up after seeing a dream where he impersonates the Stone Monkey and Doraemon, a traveler from the Western dynasty. He wakes midst his friends practicing a school play, Journey to the West. At the end, he promises to find the actual Sun Wukong and if he is unable to do so, Gian can use any gadget of Doraemon. At home, unable to find Doraemon– who is enjoying a virtual video game– Nobita travels to 645 AD where he notices Sun Wukong resembling his face and later helping a white starving kid whom he helps. Hours later, he also brings along his friends with Doraemon after which they are prevented by an uprising cavalry. They later spot Tang Seng after which Doraemon disguises Nobita as Stone Monkey through the game. During some talks, a monster secretly cradles out of the game.<br /><br />After losing his bet, Nobita and his friends return home. Gian asks for the video game and the group starts playing it, but there were no monsters in the game. Doraemon says that there might be a problem in the functioning and Gian, Shizuka and Suneo return to their homes. They find the atmosphere to be extremely dark. At night, Nobita sees that his father has horns but he thinks that he is just seeing things.<br /><br />The next day, when they were rehearsing, Dekisugi says that Shizuka (playing the role of Tang Seng) will be eaten by the monsters. The group opposes him saying that this is not the story. But slowly, Dekisugi gets angry and develops horns, seeing which the group gets shocked. Their teacher arrives, and when the group complains about the matter, the Teacher gets angry and turns into a monster. As soon as Nobita reaches home, he and Doraemon, using Bamboo Copters, set out to survey the city and they find a huge Chinese-styled building and Doraemon realizes that the monsters from the game came out when they left the machine open, yesterday, when they went to 645 AD, where the monsters came out from the machine and the course of history changed. To stop the growth and rule of monsters, the friends decide to travel back to fix things. They disguise as their respective characters and travel back in time.<br /><br />Upon reaching, the camera pans upon to Tang Seng along with the white kid, resting in a temple. After two monsters and their pets attack Tang Seng, Nobita disguised as Stone Monkey defeats them along with Doraemon and his friends. Tang Seng leaves them for his journey and the friends head forward to eliminate all monsters in Tang-Seng's way but they get exhausted and open a camping capsule during the route where they relax. At night, Tang-Seng awakes to peep the white kid whispering to a monster, but ignores and falls asleep.<br /><br />The next dawn, a flock of monster-bats carry Tang-Seng towards the castle whereas the white kid lives. Gian advises Nobita to take a look at Tang-Seng. As he moves, he passes by a past Nobita who identifies him Sun Wukong. He stops by a lake and notices the castle hosting Tang-Seng. After trying to save Tang-Seng