This lecture summarizes the nuclear accident at Fukushima Daiichi Nuclear Power Station which was caused by the big earthquake and the subsequent tsunami in Japan, 11 March 2011. The focus is on external and internal events which happened in the plant and the lessons we should learn from this accident. At first, fundamentals of nuclear engineering and BWR systems will be explained for those who are not familiar with the nuclear technology. Based on the basic knowledge, the event sequence of each reactor unit will be traced, from which lessons learned will be discussed. Finally, the current status and recovering plans will be introduced. The lecture is intended to provide validated information on the accident as widely as possible. So it is presented not only for nuclear experts but for non-experts who are interested in what happened in Fukushima. -- NOTE: This colloquium was recorded automatically and processed with minimal human intervention.