At 11:46:01, California time, on a foggy Monday in January 1986, the universe - suddenly, inexplicably, without warning - bifurcated. Fast-forward to 35 years later: Felix Sayers is a culinary writer living in San Francisco of Universe A who spends his days lunching at Coconut Café and dreaming of a successful career penning Agatha Christie-style mysteries. But everything changes when his Aunt Henrietta dies, leaving Felix a photograph of his father and himself - dated 10 days before Felix was born. It can only mean one thing: Felix has an "alter" in Universe B.

In a panic that his mystery novel may exist already, Felix crosses to San Francisco B and proceeds to flagrantly violate the rules of both worlds by snooping around his alter’s life. But when he narrowly escapes a hit-and-run, it becomes clear that someone knows he’s crossed over…and whoever it is isn’t happy about it. With the help of a trio of students and a Miss Marple-style detective, Felix must uncover the truth about his alter, the events of one Monday, and a wayward rubber duck before his time in both worlds runs out.