This is the belief of the Twelver
Shi'i:
That the true spiritual successor of the Prophet Muhammad (may peace be upon him and his family) was his son-in-law Imam Ali, husband of the lady Fatima al-Zahra. Imam Ali was succeeded in his turn by his sons Imam Hasan and Imam Husayn, and nine Imams from the lineage of Husayn thereafter. Together these fourteen, the members of the Household (Ahlul Bayt) of the Prophet (may peace be upon him), are the Masoomeen, the Infallibles, for they were blessed by Allah and purified from all sin and error for the duration of their lives.
That the last of the Twelve Imams was Muhammed ibn Hasan al-Mahdi, the Guided One who had been prophesied, also known by his titles Imamu'l-'Asr (the Imam of the Period) and Sahibu 'z-Zaman (the Lord of the Age).
That he was the son of the eleventh Imam Hasan al-Askari and the lady Nargis and that his father, knowing him to be in danger from the persecution of the Caliph, kept him hidden from the day of his birth in 869 C.E.
That upon the martyrdom of his father in 874 al-Mahdi remained hidden.
During this period, called the Lesser Occultation, he communicated with his followers through four intermediaries through whom he answered the demands and questions of the Shi'ah. The duration of the Lesser Occultation was seventy years. Upon the death of the fourth special deputy the Period of the Greater Occultation began, during which al-Mahdi has remained hidden and which endures to this day.
That al-Mahdi remains alive through the miraculous intervention of Allah until the day of his return, which has been prophesied in a hadith of the Holy Prophet (may peace be upon him):
"If there were to remain in the life of the world but one day, God would prolong that day until He sends in it a man from my community and my household. His name will be the same as my name. He will fill the earth with equity and justice as it was filled with oppression and tyranny."
That al-Mahdi has hidden himself in a cave below a mosque in Samarra, which is blocked by a gate called by the Shi'i Bab-al Ghayba or the Gate of Occultation. This is one of the most sacred sites of the Shi'i, and the faithful gather here to pray for the return of the Hidden Imam.