A.J. and Rick Simon are brothers and private investigators but are as different as night and day. A.J.'s went to law school, while Rick's a Vietnam vet, who knows a lot of people on the other side of the law, which comes handy in their line of work. While A.J. likes to do things by the book, Rick likes to do things his way.
In the show's first season, their office was in front of the office of Fowler Investigations, which is runned by Myron Fowler and his daughter, Janet, who just happens to be A.J.'s former girlfriend. Occasionally, they competed with Fowler, other times they turned to him and/or Janet for help, and vice versa.
During the second season, Fowler retired and closed his agency and Janet joined the D.A.'s office. During the third season Janet was replaced by Detective Downtown Brown, who got involved, sometimes reluctantly, with the Simons' cases and bailed them out when they got in trouble with the police.
After four seasons, Tim Reid who played Brown left the show. So he was replaced by Abby Marsh. Also Simon's mother, Cecilia, occasionally got involved with their cases or brought them some.