The Clockmaker’s workshop
In 1986, the Clockmaker has published a book about the possibility of time travel, and he has completed his apparatus. But something is missing. One day, the Stranger enters his workshop and asks him to repair an apparatus that is an exact match of his own. It is the same one — but from a different time.
The Clockmaker realizes what is missing. With the help of the smartphone that Ulrich left behind 33 years earlier, and a sample of the radioactive Cs-37 isoptope, he is finally able to activate it.