A controller acquires an excavation start position at which a work implement starts excavation. When a current landscape includes an upward slope and a downward slope existing ahead of the upward slope and the excavation start position is on the upward slope, the controller determines a first virtual design surface including a first design surface located below the current landscape and inclined at a smaller angle than the upward slope. The controller generates a command signal that causes the work implement to move along the first virtual design surface.