Pete, is that you?

“Mad Men” star Vincent Kartheiser trades the three-piece suits and slicked coif of his famous ad man character Pete Campbell for hair extensions and a scruffy beard to play Mayflower pilgrim William Bradford in National Geographic Channel’s original movie “Saints & Strangers.”

The four-hour film, which continues Monday night at 9 p.m. (part one re-airs at 7 p.m.), tells the “real story” of the first Thanksgiving that you aren’t taught in grade school — namely that contrary to popular belief, the Pilgrims who sailed to America on the Mayflower in 1620 were not all religious separatists (like Bradford) fleeing persecution.

In fact, only half the ship’s 102 passengers were seeking religious freedom (the “Saints”), the other half were pursuing adventure and financial opportunity (the “Strangers”), which often led to clashing values on how to build their new settlement in Plymouth, Mass.

“It was a challenge for me to undertake this role and find that level to play him at because the role is very modest and very devout,” Kartheiser says of portraying Bradford, leader of the Saints. “I’m used to playing a character that is much louder and brasher … more reactionary and mischievous and this character is not that.

“He’s steady and thoughtful.”

Bradford traveled to the New World with his wife, Dorothy (Anna Camp, “Pitch Perfect”), though they left their three-year-old son behind in Holland — which may have contributed to her falling (or jumping) off the Mayflower and drowning while her husband was on land exploring.

“He didn’t write anything about her death. That’s why they don’t know if she committed suicide or if it was an accident,”Camp tells The Post. “They think it might have been of her own accord because it was the highest sin to take your own life. That’s how I wanted to play it.”

William would go on to serve as one of Plymouth’s earliest governors, playing a key role in the founding of the colony along with first governor John Carter (Ron Livingston, “Office Space”), designated Native American diplomat Edward Winslow (Barry Sloane, “Revenge”) and military advisor Myles Standish (Michael Jibson).

After beginning with the hardships the Pilgrims faced at sea and on land as they built their colony, night two of “Saints & Strangers” focuses on the settlers’ fragile alliance with the Native American Pokanoket tribe and their leader, Massasoit (Raoul Trujillo), through betrayal and attacks.

“Something that will stand out to a lot of people was why did the Native Americans allow these people to survive?” Kartheiser says of the English colonists who took goods, brought disease and killed natives. “That’s something we explore — the interpolitics of the tribes of the region and the politics of this specific group of people. We reveal a little bit about what was going on and why Massasoit chose to help them.”