- Read my post, which came a few minutes before tomndebb’s.
Once upon a time I wrote a huge post on Celestine V here, but I think it was lost during the Great Board Crash of two-thousand-and-something. Here’s the gist of it, though:
Pietro di Murrone was a hermetic, ascetic monk who roamed the hills around Aquila. When Pope Nicholas IV died in 1292, political feuding prevented the Sacred College from electing a new pope for two years. Finally acknowledging the deadlock, the cardinals agreed upon the elderly Pietro, thinking that the aged and reclusive priest would be a safe choice to head the Church while they continued their corrupt politicking. The saintly country man, they reasoned, would be completely lost in the bureaucracy of Rome, and would be easy to manipulate into doing what they wanted. Besides, he was so old that he would soon be dead, by which time each faction thought it would be able to gain the upper hand for the next papal election.
The news of his election came as a complete surprise to Pietro. He was not even a bishop at the time, and had no idea he was in the running. He at first refused the office, but a delegation from Rome was dispatched to his mountain enclave to beg him to accept the honour. It was joined by hundreds of thousands of common folk from all around Italy. Seeing such an earnest show of support, Pietro reluctantly acquiesed, and left his life of fasting and prayer to receive the papal tiara as Celestine V.
Contrary to the expectations of the conclave that elected him, the new pope immediately set about appointing new cardinals and bishops, instituting and abolishing laws, reforming monastic orders, and so on. These acts caused huge upheavals within the Church. His successors later claimed that he was simply incompetent, but there are those who believe that Celestine was fully aware of what he was doing and was trying to undo much of the corruption and centralization in the Church. Whatever the reason, the frequency and speed at which the reforms were instituted earned him the enmity of the Church’s upper echelons, and soon the cardinals were plotting of how to rid themselves of him. Cardinal Benedetto Caetani, an expert in canon law, was appointed to determine whether it was possible for a pope to resign. After serious deliberation over the question of whether there was, in fact, any power on earth who is authorized to accept such a resignation, Caetani eventually decided in the affirmative. Soon after, rumours (originating with the cardinals, no doubt) that the pope was to resign began spreading throughout Italy.
Because Celestine daily faced the hatred of his immediate subordinates, and because he dearly missed his quiet life of peace and contemplation in the mountains, he did not deny the rumours, and within a short time formally announced his intention to abdicate. The common people and lower clergy, who dearly loved him, were shocked and upset, and throngs of them surrounded the papal castle in Naples imploring him to stay. But Celestine had made up his mind, and resigned. His successor was none other than Benedetto Caetani, who won the election almost immediately. Caetani, now Boniface VIII, triumphantly marched to Rome with Celestine in tow. He officially annulled practically every decree every issued by his predecessor and formally dissolved the branch of Benedictine monks he founded. Most of Celestine’s acts were expunged from the official record books, and the man himself was forcibly imprisoned “for his own protection”. In reality, Boniface and the other cardinals were fearful that Catholics would not accept the regime change and would rally around Celestine.
Eventually, Celestine managed to effect his escape from Rome, and, to the great joy of his fellow monks, appeared back at his mountain abode. Boniface was furious and declared him a fugitive. The 90-year-old Celestine attempted to flee across the sea, but he was captured and returned to Rome. There Boniface imprisoned him in a tiny cell high in a tower. After almost a year of mistreatment, the old man died, alone and unhappy.
There are some interesting postscripts and asides to this story. One is that Celestine may, in fact, actually have been murdered by Boniface. If I recall correctly, Celestine’s remains were stolen from their resting place sometime in the last hundred years. When the bones were eventually recovered, it was noticed that there was a large hole in the skull, lending credence to an old rumour that his successor had him quietly dispatched with a nail in the head. However, a Turin scientist was appointed to study the skull in 2000, and he declared that the hole was made after death.
Another interesting anecdote is that Celestine’s decision to resign was “helped” rather directly by Caetani himself, who hid behind an altar during one of Celestine’s private prayer sessions and impersonated the voice of God, saying, “Resign! Resign!”. Seems unlikely that anyone would be fooled by this sort of thing, but it makes for an amusing scene. 
Yet another interesting tidbit is that while Celestine V was eventually canonized, Dante (in his Inferno) placed the former pope squarely in Hell for his abdication. I wonder whether he was writing before the canonization, or if his patron was one of Celestine’s enemies, or whether he just didn’t agree with the thought of a pope throwing in the towel.