The oxygen tanks had to be stirred periodically to obtain more accurate readings of how much gas was left. Unfortunately, two days into the journey, a routine stir of an oxygen tank in the Service Module ignited a wire inside it that had damaged insulation. This caused an explosion that emptied the contents of the Service Module’s oxygen tanks to space.
The loss of oxygen became a dire emergency: the oxygen in the Service Module was needed not only for the astronauts to breathe, but also for power generation and propulsion. So, the plan to land on the moon was scrapped. Instead, the crew looped around the Moon for a trans-Earth injection so that they could head back to Earth, a journey that would take four more days.
Now that the Service Module was out of action, the astronauts shut down the Command Module's systems to conserve its remaining resources needed for re-entering the Earth. They transferred to the Lunar Module, originally meant to be used to land on the moon. It only had enough oxygen for two astronauts for two days. But, now, it had to support three astronauts for four days! With help from Ground Control, they modified the Command Module’s carbon dioxide scrubber system to work in the Lunar Module so that the oxygen would last longer.
After enduring four days of great hardship, caused by limited power, a chilly and wet cabin and a shortage of drinkable water, the astronauts safely returned to the Earth on April 17th. The astronauts in the Command Module splashing into the South Pacific ocean was watched on TV by tens of millions of people. The ordeal was chronicled in a memoir co-written by one of the astronauts, Jim Lovell, called “Lost Moon”, and later made into a movie.