A man places a classified ad looking for somebody to travel through time with him. You probably don’t need to hear much more before reaching a conclusion.

He’s a crackpot.

Safety Not Guaranteed gives you plenty of reasons to think so. What makes it interesting is watching how your opinion changes once you spend some time with the guy. You don’t necessarily become convinced he can travel through time. But somewhere along the way, you realize you’re rooting for him.

And you’d rather not see him humiliated.

This Should Be an Easy Story
The setup practically writes the magazine article for you. A man named Kenneth has placed an ad seeking somebody to accompany him on a trip through time. A Seattle magazine sends a writer and two interns to investigate. Kenneth, played by Mark Duplass, is a supermarket employee who takes his proposed expedition very seriously.
Aubrey Plaza plays Darius, the intern who approaches Kenneth by pretending she wants to join him. She’s unimpressed with much of her own life, so you wouldn’t expect her to be especially receptive to whatever this stranger is selling.
Kenneth doesn’t make it easy, either. He’s suspicious, demanding and concerned about preparations that seem excessive for a trip you doubt will ever happen. There’s training involved. Apparently, responding to the ad doesn’t automatically qualify you for departure.
It’s funny because he’s so serious. But that seriousness also makes it increasingly difficult to dismiss him.
Duplass Gives You Somebody to Care About
Kenneth could have been an exhausting character. Duplass makes him much more watchable than that. Kenneth can be awkward and unreasonable, but there’s a sincerity underneath it that keeps the performance from becoming a collection of odd behaviors. 
He isn’t trying to entertain the people around him. He has something important to do, and he wants somebody he can trust beside him.
You can laugh at the preparations while recognizing how much they mean to him. And once you recognize that, Darius’s deception starts to feel less comfortable. What initially seemed like a harmless way to get a story now involves a person who doesn’t know he’s part of the joke.
Plaza handles that changing relationship well. Her dry delivery gives the early scenes their humor, but she also lets you see Darius becoming interested despite herself. Their conversations gradually feel less like an interview and more like two people finding somebody they actually want to talk to.
You Still Want to Know Whether It Works
Of course, the time machine matters. A movie can’t introduce that possibility and expect you to forget about it because the characters are getting along.
But Safety Not Guaranteed gives you another reason to stay interested while you wait for answers. You start wondering what will happen to Kenneth if Darius lets him down, or if the expedition he’s invested so much in amounts to nothing.
That’s a good position for a movie to put you in. Your curiosity gets you through the door; your concern for somebody keeps you there. Eventually, you’re hoping she doesn’t turn out to be one more person who laughs at him.