Star Trek The Next Generation Times Arrow

An engineering team finds evidence of an alien presence on Earth in 19th century San Francisco Datas severed head buried five hundred years ago An investigation inadvertently sends Data back to 19th century San Francisco where he meets Jack London Mark Twain and a younger version of the long lived Guinan Following Data back in time the crew of the Enterprise must locate Data and prevent his death while stopping the enigmatic Devidians from destroying Earths history