Following the events of the previous games, youkai soon begin to swarm the Hakurei Shrine, prompting Reimu Hakurei and Marisa Kirisame to separately head for a lake in the mountains, which appears to be the source of a tremendous power surge. The two reach the gateway underneath the lake, which teleports them to a strange Fantasy World, in which the mansion Mugenkan exists, where the mastermind supposedly is. (Touhou Wiki)