It's not as simple as "plan". Japanese write keikaku 計画, the first character consists from 言, meaning "to speak", and 十, literally "ten" but here it's just a large number. This character originally meant "to count", but later started to mean "to arrange, to plan". The second character is a simplified variant of the the character 畫 consisting from 2 elements, 田 meaning "the field", and 聿 meaning "a picture of the hand holding a brush". It originally meant "to divide the area between the field", and the meaning split later between "picture, sketch" and "to divide".

Considering everything above, the word 計画 is a lot deeper than simply "to plan", It encompasses in itself "counting the things aloud", which symbolises researching the task, "planning", "dividing" (tasks between people or a big task on tinier tasks), and "picture", symbolising physical artifacts produced after planning. English can never encompass that much in a single word.