In an email addressed to employees that was obtained by ABC News, Plepler described the hack as a "cyber incident directed at the company which has resulted in some stolen proprietary information, including some of our programming."
"Any intrusion of this nature is obviously disruptive, unsettling and disturbing for all of us," he wrote.
Plepler added that HBO's "senior leadership and our extraordinary technology team" are working to "protect our collective interests."
"As has been the case with any challenge we have ever faced, I have absolutely no doubt that we will navigate our way through this successfully," the message concluded.
A separate statement, obtained by ABC News, revealed more details about the hack, including that law enforcement is investigating it.
"HBO recently experienced a cyber incident, which resulted in the compromise of proprietary information," the statement began. "We immediately began investigating the incident and are working with law enforcement and outside cybersecurity firms. Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold."
Entertainment Weekly reports that the hackers obtained 1.5 terabytes of data, including a script for "Game of Thrones." The hackers, according to EW, also got their hands on an upcoming episode of "Ballers" and "Room 104," which have been placed online.
A representative for HBO wouldn't confirm what programming was affected by the hack.
"We cannot elaborate off the statements due to an ongoing investigation," a company representative told ABC News.