The hero stumbles upon a murder, and then another, then another... The only problem is, there doesn't seem to be any connection between the victims.
The first step in solving any murder usually involves figuring out who the victim's enemies were: Who would want the victim dead? The problem is, these victims are all (apparently) completely unrelated. There isn't anyone who would want all of them dead because there isn't anyone who knew all of them.
Often in these mysteries, discovering the connection between the victims is the key to solving the case. Sometimes, however, the connection isn't discovered until after the murderer is caught. In some cases, the hero never discovers the connection, and the killer has to explain it after he's caught.
Basically, there *is* a connection between all the victims, but the connection is so strange and obscure it takes detectives a long time to find it.
NOTE: Since this is a Mystery Trope, there may be unmarked spoilers.
- There's a Ms. Tree story where several men are murdered. Aside from having all attended the same school, there's no connection between the victims... until it's discovered that at a rowdy party they once all got drunk and gang-raped a girl. Since few people knew about the gang-rape, there wasn't anything to connect them to each other.
- The Last Kashmiri Rose involves a string of murders of English women in India, but there's no other obvious connection. It turns out that each victim was the wife of one of a group of officers who the killer held responsible for the death of his own true love.
- In Point of Contact by J.T. Edson, a murderer hits a string of seemingly unconnected people: different sexes, ages, occupations, social classes, etc. The connection turns out to be they all served on the same jury.
- Done as a Reverse Whodunnit in Donald E. Westlake's "The Ax", where the murderer depends on the lack of any obvious connection between his victims.
- Joe Pickett: In Blood Trail, it looks as if there is a Serial Killer targeting hunters. However, Joe eventually discovers that there is a connection between the victims aside from hunting. They were all members of a specific hunting party 12 years ago, and the murders all connect to an event that occurred on that trip.
- Stella Nickell poisoned her husband by lacing Excedrin capsules with cyanide, and putting poisoned boxes back on drugstore shelves, which also killed 15-year-old Susan Snow. Nickell attempted to make her husband's murder look like a random occurrence among other deaths.
- Ronald Clark OBryan poisoned his son and attempted to poison his daughter for insurance money by giving them and three trick-or-treating children potassium cyanide-laced Pixy Stix on Halloween. The other children didn't eat the candy.