'20/20' FEATURES INTERVIEW WITH MAN WHOSE WIFE SECRETLY RECORDED HIS AFFAIR AND THEN SNAPPED, KILLING HIS GIRLFRIEND AND THEN DYING BY SUICIDE

He Discusses His Marriage, Affair and How His Wife Turned Into a Killer

'20/20' Airs on the West Coast, Saturday, Dec. 21 (10:00 - 11:00 p.m. PST), on ABC

"20/20" features interview with Mark Gerardot.

"20/20" co-anchor Amy Robach sits down for an interview with Mark Gerardot, whose wife, Jennair Gerardot, killed his girlfriend and former boss, Meredith Chapman, and then died by suicide. "20/20" reports on Jennair's obsession with Mark and Meredith's relationship, including details on how she meticulously sewed listening devices into his clothes to record his conversations. In his interview, Mark discusses his 24-year marriage to Jennair and their lives together; his whirlwind relationship with Meredith; his wife's murder plot; and his life now, including his quest to figure out how Jennair turned into a killer. "20/20" airs only on the West Coast Saturday, Dec. 21 (10:00 - 11:00 p.m. PST), on ABC.

The program features interviews with Michael Hartman, Mark's best friend since high school; Erin McCarthy, The Philadelphia Inquirer reporter who covered the story; Sheila Brennan, the divorce coach hired by Jennair; Detective T.J. Schreiber, Sergeant Christopher Four and Superintendent Christopher Flanagan, members of the Radnor Police Department who worked on the case; Tim Mulvey, Meredith's former neighbor; and Dr. Janice Harper, professional writing coach who is working with Mark on a book about the tragedy. "20/20" also features audio from the listening devices used in Jennair's surveillance.

Mark first met Meredith during an interview for a position at the University of Delaware. After he took the job, the two became romantically involved. Jennair became suspicious, placing tracking devices on Mark and Meredith's cars, and became obsessed with the couple.

Mark eventually confessed to his affair, and the couple attended counseling in what he thought was an effort to achieve an amicable divorce. Mark and Meredith continued to see each other, and Meredith took a new job as an assistant vice president at Villanova University. But during this time, Jennair secretly plotted Meredith's murder; and on April 23, 2018, she broke into Meredith's new home on Philadelphia's Main Line and shot her before dying by suicide. Mark discovered the two bodies. (Rebroadcast. ABC OAD: 9/6/19).

"20/20" is anchored by David Muir and Amy Robach. David Sloan is senior executive producer.