Overlord is based on (incase you haven't guessed), the Normandy beach landings, specifically the days immediately before, during and after.
Capture point one is based around the night before the d-day landings, where troops parachuted behind enemy lines to assault key targets, in this case, an axis seaside base. The allies must infiltrate the base and 'capture' the grappling hook in order to ascend to the higher levels of the base and destroy the main objective, two seaward facing guns.
Capture point two is the landing and the omaha beach itself the next morning, where the allied objective is to move up the beach and break the seawall with a detpack. After this they must move the flag into the axis base and onto the capture point in the basement.
Capture point three is set in a town a few days after the landings that was bombed the previous night (Caen), with some of the fires still raging in the areas hit. The allies must move through the town through several choke points and then place the flag on the capture point at the top of a hill to win the game!
Taken from the readme:
Attack, Defend, Normal and reverse Capture the Flag. Overlord is all of the above, a large 'theatre of war' map for tfc based around the overlord campaigns before, during, and immediately after the D-Day landings.