San Francisco — Familiarity can breed either contempt or comfort in a new TV series. The passage of time can help nudge a new show like “Unforgettable” toward the latter category, and having an appealing actress in the lead doesn’t hurt either.

The show, which kicked off last week on CBS and based on a story by J. Robert Lennon, is about an ex-cop named Carrie Wells (Poppy Montgomery, “Without a Trace”) who is haunted by the murder of her older sister when they were both kids.

Anyone would be affected by a tragedy of that magnitude, but it’s particularly difficult for Carrie because she has an extremely rare condition known as highly superior autobiographical memory, or HSAM. Only a handful of people in the country, including the actress Marilu Henner, who is a consultant on the show, have the condition, also called hyperthymesia.

Carrie, like Henner in real life, can recall virtually any detail of any day in her life. While that may sound kind of cool at first, consider what it must be like living with that much information, so much of it beyond mundane: Your head must feel like it’s exploding with facts. If someone were to ask you where you were at 4 p.m. on Wednesday, Nov. 3, 1995, would you know? Someone with HSAM would be able to tell you about the weather that day, the people who passed them on the street, which scarf their mother was wearing when they picked her up for her dental appointment at 4:10 p.m. and even that Nov. 3, 1995, was a Friday.

Hyperthymesia is the gimmick, if you will, that separates “Unforgettable” from the old NBC police procedural, “The Profiler,” which starred Ally Walker. Police profilers are skilled at putting psychological pieces together to create an informational picture of a criminal. But someone with hyperthymesia who happens to discover the body of her upstairs neighbor bleeding out in the alley behind their Queens apartment building is able to revisit her own highly detailed memory of that moment and other moments in her recent past to figure out who murdered the woman.

Carrie became a cop in order to find out who killed her older sister, Rachel. She has revisited the autumn forest where she and Rachel were playing that day thousands of times in her head. But despite having hyperthymesia, she has been heretofore unable to re-see herself discovering Rachel’s body.

The other plot-propelling gimmick of the show is that Carrie has left not only her career as a cop back in Syracuse, but her former boyfriend as well. Detective Al Burns (Dylan Walsh) just happens to have taken a job in New York and — wouldn’t you know it? — is the lead investigator on the neighbor’s murder. Well, by the time we’re asked to believe that coincidence, show creators John Bellucci and Ed Redlich have already helped us wrap our minds around hyperthymesia, so we’re pretty open to anything, at least momentarily.

Although hyperthymesia is a gimmick, it works. In fact, the coolest thing about the show (beyond Montgomery’s infectious likability) is watching Carrie watch herself in the past, see again what she saw the moment she found the neighbor’s body, and even an earlier moment when she was walking back to her apartment. As Carrie revisits these times, we revisit them as well. Knowing what she knows, that a woman will be stabbed to death, we see what she sees as she rewinds the tape of her own experience.

Perfect recall of the past may be enough to ensure a future for “Unforgettable.”

‘Unforgettable’

When: 10 tonight

Where: CBS