The Flying Doctors (1986) : 6x19

Nick and Annie find a car in a dried creek with two bodies inside. They are believed to be Bob and Rachel Jordan, who mysteriously disappeared years earlier. Tom is suspicious and finally they find out the man in the car wasn't Bob. It turned out Rachel had had an affair - Bob was ill and spent most of his time in bed. Their son found out and told Bob, who was very upset. Rachel and her ""boyfriend"" took off, but apparently they didn't get any further than the creek. Shortly afterwords Bob died, but made Michael swear not to tell the truth. Michael held his promise and burried Bob at the property. He had had mental problems ever since.