Jim Lovell, commander of the mission, was looking forward to his time on the moon. So was Fred Haise. On the other hand, Ken Mattingly (originally slated as the third member of the crew) was still home in Houston. He'd been exposed to German measles and, 72 hours before launch, was replaced by Jack Swigert. Turns out, it was a good thing Mattingly was still on the ground. His expert help was going to be needed by his crew mates. (This link takes you to the official portrait of the crew which includes Mattingly in the middle.)

Fifty-six hours into the flight, Apollo 13 was in serious trouble. There was a short in the current from fuel cell three. That current was supplying power to the cryogenic oxygen tank. Two other shorts occurred a few seconds later. Not only was the mission in trouble, the lives of the crew were in danger. The severely damaged Service Module had lost its ability to produce oxygen and water. Lovell radioed mission control:

Mission Control in Houston, together with Ken Mattingly, feverishly worked out a way to get the crew back to earth. It wasn't easy. The solution was complicated.