Immanuel Velikovsky was a psychotherapist who, working far outside of his professional field, developed and published two intertwining sets of controversial theories, which have been accepted and believed by many thousands who have read his books.
In one set of theories, he saw the Solar System as having experienced colossal upheavals within historical times. These upheavals included close encountrs between the Earth and, on different occasions, Venus and Mars, before the planets settled into their present apparently stable orbits. One of those encounters, he proposed, was responsible for the series of catastrophes recorded in the Old Testament book of Exodus as the Ten Plagues that fell on Egypt.
The other set of theories derived from his realisation that an ancient Egyptian document (which he called the Papyrus Ipuwer) could be interpreted as giving an Egyptian account of the same series of catastrophes. The difficulty was that historians and archaeologists placed the two accounts--Ipuwer and Exodus--several centuries apart. By aligning the two together, and seeking a period of Egyptian history into which the accounts might fit, he brought about a "Lost Synchronism" between Biblical and Ancient Egyptian history.
The Old Testament described numerous interactions between the people of Egypt and the people of Israel, but archaeologists had been unable to find anything much in Egyptian history that convincingly parallelled the Bibiolical records. Velikovsky's re-synchronisation of the two timelines appeared to reveal the missing Egyptian accounts of those interactions, in startling richness of colour and detail. To make it work, however, he had to find a way to excise something like 500 years from Egyptian history, as it had been reconstructed by 19th-Century historians and archaeologists out of the fragments left to us from ancient times.
The outcome of these two sets of theories was two parallel series of books. Those which give detailed accounts of the supposed interplanetary catastrophes are not of my concern on this seb site. My interest has always lain, instead, in the books which set out his reworking of ancient history.