Ruby - Rails - Refactoring

How might one refactor this bit of Ruby on Rails code?

    def select_plan
        unless params[:plan] && (params[:plan] == '1' || params[:plan] == '2' || params[:plan] == '3' || params[:plan] == '4' || params[:plan] == '5' || params[:plan] == '6' || params[:plan] == '7' || params[:plan] == '8')
            flash[:notice] = "Please select a membership plan to register."
            redirect_to root_url
        end
    end

Thanks