I'll try to keep this short. In 1915, the UK was at war with Turkey, among other Central Powers. The planned to take Turkey's capitol, Istanbul, by first demolishing it was a bombardment from the sea, then by an invasion. This couldn't be done unless the Turkish guns guarding the approach to Istanbul were silenced. To this end, the British attempted an amphibious landing on the southern end of the Gallipoli peninsula and the Australian/New Zealand troops landed ten miles to the north. Their joint objective was a hill named Achi Baba, about ten miles inland, from which fire could be laid down on the Turkish guns.
The invasion failed. The Brits in the south were laid waste by a few Turkish machine guns. The Anzacs landed against only sniper fire but their attack stalled a short way inland. The battle then turned into what the British wanted least -- a replication of the trench warfare in Europe. All of the British supplies had to be lugged in from the beach, and some of the drinking water came from as far away as Egypt.
The spirit of the Turkish troops had been underestimated. They were outnumbered but they were fighting for their homeland and were inspired by their leader, Kemal Ataturk. Disease sickened and killed many of the British and there was evidence of slack leadership. After eight months of combat, they were no closer to Achi Baba -- which was to be taken the first night of the invasion -- than about half way. They finally withdrew.
It was a great victory for the Turks and an awful defeat for the UK, for which Winston Churchill, First Lord of the Admiralty, was largely responsible. He resigned shortly after, and it may have been the specter of Gallipoli that partly explains his reluctance to mount a cross-channel invasion in World War II.
However, while acknowledging all these forces, the program examines one of the more neglected factors of the affair -- the absence of adequate intelligence about the terrain and the Turkish position. The Anzacs got ashore, for instance, and carried their advance inland but when they moved to take the heights that were their objective, they found that the river and flat plain on their maps were in actuality a kind of miniature African rift valley with vertical sides. The flat beaches and gentle slopes of the British landing on the southern point turned out to look more like Omaha Beach, with well-emplaced machine guns on each flank.
Like many battles it was horrible and a material loss for both sides. The Turks could claim a victory in having repelled the invaders, but it cost them dearly. The British and Anzacks couldn't even claim a moral victory. As Elizabeth I is supposed to have said, "I dislike wars. They have unintended consequences."