Having watched just about every documentary there is and read every book I could find I'd like to think I have a fairly good layman's understanding of what happened on that morning in April 1986. However, there are a lot of very detailed descriptions out there on the web and I don't see a lot of value in me trying to replicate them here. Instead what I will do is give a very high level introduction to the disaster and then point you in the direction of the best reading and viewing.
One of the other reactor control rooms at Chernobyl - identical to that of Reactor 4
Keeping it as simple as possible, RBMK reactors use water as a coolant. This coolant is driven around the reactor core by pumps, which of course require electricity. As such, worries existed over what would happen if there was a loss of power. The theory was that residual momentum from turbines winding down could be used to generate enough electricity to drive the coolant until the back-up generators kicked in. If it couldn't then the possibility existed that that core may be damaged due to a lack of coolant. This scenario had been tested at least three times before and on each occasion it had failed to produce the desired results. However, further modifications now led to a fourth test, using Reactor 4. The man in charge of the fateful shift should have been Alexander Akimov, but on this particular night he was outranked by the plant's deputy chief engineer, Anatoly Dyatlov.
At just after 01:23 the test commenced as turbine stop valves were closed and the pumps powered by the turbines started to run down. As history tells the test didn't go as expected and instead an already unstable reactor began to increase its output rapidly. Alexander Akimov then took the decision to press the AZ (SCRAM) button to trigger an emergency shutdown. This should have seen all of the control rods safely inserted into the core thus slowing and eventually stopping the fission process. Instead it resulted in a power surge which led to the first explosion. Moments later a more damaging second explosion completely destroyed reactor number 4.
Reactor control panel with the AZ (SCRAM) button in the middle of the top row
Within seconds of the disaster unfolding the fire alarm was activated. The turbine hall roof was on fire and first on the scene was the crew of Commander Vladimir Pravik, the plants own fire station. Soon calls were going out to fire stations in Pripyat, Ivankiv and Poliske. A sample of these frantic calls can be heard in the chilling recording below.
All fires were out by dawn, apart from of course the fire in the reactor itself which would burn for many more days. Tragically the first firemen on the scene were also amongst the disaster's first victims. Vladimir Pravik and several of his men climbed onto rooftops surrounding the destroyed reactor to deal with multiple fires. The lethal doses of radiation they received saw them die unimaginably cruel and painful deaths within a matter of weeks.
Liquidators at work decontaminating the areas worst affected by fallout
With the fires around the reactor out the next challenge was the reactor itself. For this no amount of fireman would do. Shortly the Soviet Union's leading experts on the nuclear industry were on site. Leading the scientists was Valery Legasov. Leading the politicians was Boris Scherbina, the Deputy Chairman of the USSR Council of Ministers. Their task was to prevent an even bigger disaster following on from the destruction of Reactor 4.
Sand mixed with Boron and Dolomite was used to cover the exposed radioactive material which continued to burn and give out highly radioactive and toxic pollutants into the air around the plant. Another potentially devastating explosion was avoided when water beneath the reactor was drained. A tunnel was then dug by local miners to allow construction of a new concrete floor under the reactor core. This was to prevent the local water table from becoming polluted. These actions were only the start. Many thousands of "liquidators" risked their lives to ensure that the decontamination process was carried out as well as humanely possible. Their sacrifice is one large parts of Europe should be eternally grateful for.