First I saw this plant in my neighbor’s entrance garden, a narrow space with begonias and lillies packed tightly. A few days later, I bought the one below. It seems like this variety of clematis is very popular in Tokyo this summer. I’ll have to pot it up soon.
Thanks to @Jencjoyous from Napa, California, for identifying this variety as Clematis ‘Nelly Moser.’ In Japan, it goes by the name Dr Rapperu ドクターラッペル.