Twenty years ago, seven detective-story fans formed a little group called the Seven Unravellers. A London bobby, a solicitor's clerk, a baronet, an ex-Army major, a greengrocer's daughter, a chemistry student, and a bohemian eccentric. They were as mixed a bag as any group of genuine murder suspects. But they had one thing in common: murder. Not the fact but the fiction of it. Once every month they met to discuss, and possibly solve, the latest crimes in murder fiction.

Now the Unravellers are together again, linked by the bizarre murders of three of their members - one of the deaths is the classic "locked-room" mystery in reverse.

The series of clues left behind are as puzzling as the crimes themselves. Each is related to one of the colors of the spectrum - the red dye in the sea near one member's home, the orange thrown through the window of another's office, a page from the post office's Yellow Pages pinned to a third member's door. At the first sign of trouble, amateur detective Thackeray Quin is called in.

Invisible Green is a brilliant detective story in the classical mold, but with a puzzle to tax the most up-to-date minds. John Sladek's plotting is precise, his suspects and victims from the dotty old Major to the surly ex-policeman and the trendy art director eccentric and entertaining, and the mystery highly ingenious.