The second in command of the executive branch of the U.S. government is the vice president. He is the one who takes over the reigns of government if the president dies, steps down or becomes incapacitated. Although vice-presidential powers are fairly limited otherwise, this duty is critical.
The rest of the executive branch consists of the Cabinet members. The secretary of state is the top-ranking Cabinet member but is several places down the presidential line of succession. After the vice president, the speaker of the House and the president pro tempore of the Senate are next in line before the secretary of state.