Survivor of World’s Worst Tsunami Remembers Moment He and His Little Brother Lost Their Parents on the Beach

"I don't mind telling the story of what happened to me on the day," Louis Mullan tells PEOPLE in this week's issue

• Louis Mullan and his brother Theo last saw their parents on the beach in Khao Lak, Thailand just before the historic Dec. 26, 2004, tsunami brought devastation

• "We flew back on the first of Jan without Mum and Dad," Louis tells PEOPLE

• Leonard and Catherine were found dead weeks later, among the nearly 230,000 people killed in the disaster

Louis Mullan still recalls his dad telling him and his brother Theo that there was "something happening" in the water on the morning of Dec. 26, 2004, while the family of four vacationed in Khao Lak, Thailand.

"The water had retreated, and [there were] a lot of confused faces. People really didn't know what was going on," he tells PEOPLE in this week's issue, describing what it was like to see the ocean water retreat as a 9.1 magnitude earthquake off the coast of Indonesia triggered the world's deadliest tsunami.

Remembering that he'd left their room unlocked, Louis, then 16, says he ran back and was followed by his younger brother Theo, 11.

At the time, the U.K. native says their parents were "looking at the coast with all the other people." But by the time they got back to their room, "the mood had changed" and everyone was "running in the other direction."

Once they got back to the waterfront, they joined others running, but couldn't locate their parents, Leonard and Catherine. Then, the water surged.

"You could see it coming through the trees, through the buildings," says Louis, now 36. "The water catches up with you and sweeps you off your feet. I was holding onto Theo, but then after a while, the strength of water just pulled us apart."

Related: Survivors Remember World's Deadliest Tsunami, Which Killed Over 225,000: 'Everybody Lost Somebody' (Exclusive)

Louis says it felt like he was tumbling through a "washing machine." He adds, "I had run out of oxygen in my lungs, I was done. And at that very point, I did resurface and managed to get a lung of air." As he got further inland, he came across a building under construction.

"I actually pushed myself up or used a Coca-Cola vending machine as a float to get up and then managed to grab onto scaffolding," he says. "There was already a few other people in that building, [and] they grabbed me and pulled me in."

For more on life 20 years after the 2004 tsunami, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here.

Amid fears of another wave, Louis went into another building as he continued to look for Theo. Fortunately, he came across a group who knew where he was.

"They said, 'Yeah, we've got a young boy at that kind of age in our group,' " says Louis, who appears in National Geographic’s Tsunami: Race Against Time (streaming now on Disney+ and Hulu). "They brought him up and yeah, I remember seeing him for the first time ... it was incredible."

Following that emotional reunion, the brothers piled in the back of a pickup truck and spent the next two days, including Theo's 12th birthday, in the hospital.

Related: Supermodel Petra Nemcova Almost Died in the World's Worst Tsunami. How She Overcame Fear of Water and Healed (Exclusive)

"We flew back on the first of Jan without Mum and Dad. Yeah, we were just in shock," he recalls. Louis says common sense told them they weren't alive. Weeks later, Leonard and Catherine were found dead, two of the staggering 230,000 people killed in the historic disaster.

Louis says he often thinks of his parents, who would be in their 70s now. "Mad to think, obviously [that's] kind of where our memory of them stops," he adds.

Following the tragedy, the boys were taken in by a neighboring family, who raised them with "love and support."

Twenty years later, the married man, and soon-to-be father, says he doesn't think he and his brother Theo, now 31, have any special resiliency compared to other survivors. He notes, "Obviously we went through it and anyone would be similar."

"I don't mind telling the story of what happened to me on the day," he adds. "There's probably something therapeutic about it."