Admetus had been a guest of Hercules when he was traveling through Thessaly. Admetus treated Hercules with great hospitality, and Hercules promised to repay him for his kindness.
Some time later, Admetus was chosen by Apollo to be the only mortal who would be allowed to die in Apollo's place. Admetus was devastated, but he agreed to the terms.
His wife Alcestis was heartbroken, but she agreed to die in her husband's place. As she was about to be buried, Hercules arrived and fought with Death for her life. Hercules was victorious, and he brought Alcestis back to life.
Hercules then returned Alcestis to Admetus, and the two of them lived happily ever after.