Although Jimmy Valentine has just been released from prison after having served a sentence for breaking into safes, he's soon up to his old tricks again. The man's a professional safe-cracker; it's what he does best. And no sooner is he back on the street than he's picked up where he left off.
There are numerous pieces of evidence in the story to show that Jimmy's fallen back into his old ways. First of all, he goes back to the place where he was arrested—a small room above a restaurant—to retrieve the bag of expensive tools he always uses to crack open safes. After he leaves the restaurant, Jimmy takes the bag of tools with him; he wouldn't have done that if he had no use for it.
Soon, a number of safes in the Midwest have been cracked wide open, their contents stolen. We know that Jimmy's responsible because all these robberies show the unmistakable signs of his trademark style. As Ben Price, the cop on Jimmy's case, says:
I can see that Jim Valentine has been here. He is in business again. Look at the way he opened this one. Everything easy, everything clean. He is the only man who has the tools to do it. And he is the only man who knows how to use tools like this.