Joseph, Jules, and Albert (Humphrey Bogart, Peter Ustinov and Aldo Ray, respectively) have just escaped the prison on Devil's Island, but they haven't made it off the island yet. Trying to lay low until an opportunity arises, they finagle their way into the local general store, first as messengers, then by offering to fix the roof.
The first 20-25 minutes, I wondered if the movie touting Bogart was a bait-and-switch, because what he and the other two do is mostly peer through the skylights at the trials and tribulations of the family running the store. The manager is ineffectual and indecisive, worried his cousin Andre (who owns the entire chain of stores and is played with extreme stiffness and arrogance by Basil Rathbone) will sack him. The daughter is mooning over the Andre's son Paul, who she parted from a year ago, but is sure still loves her. The wife loves her husband, but is frustrated with how life has turned out.
Oh, and Andre and Paul are on the steamship in the harbor, and will be inspecting the store's finances, just as soon as they get around the quarantine.
So, what starts as the convicts trying to help the store make sales because it will give them more money to swipe when they make their escape, gradually becomes them helping the family have a lovely Christmas, and then trying to protect the family from first, Andre, then Paul (John Baer's babyface conceals just as heartless a skinflint as Andre.)
Bogart takes the fast-talking spiel he uses when laying everything out at the end of a detective movie, and spins it into a con artist who can get a bald man to spend 117 francs on a sterling silver comb set. It'll be a great thing to leave to your family! But I have no family. That's even better! Just constantly twisting and turning until the guy's head is spinning.
Ustinov is the funniest, whether because he got the best lines, or possibly just how he delivers them. Somewhere between a sadsack and upbeat, soft spoken either way. Not optimistic, but not miserable, either, despite their circumstances. Since his character was apparently a safecracker (albeit I think he's in prison for murder after catching his wife in bed with another man), Ustinov also breaks into various lockboxes, safes and locked doors with movements not unlike someone trying to work a pinball machine without getting "TILT."
The ending is probably a concession to the rules against allowing criminals to be seen as benefiting from their crimes in movies. In-story, you could probably attribute it to them being institutionalized. Though their criminal past is played for gags at times (Bogart says he owned a factory selling canned air), the movie reminds you periodically that Ray's character beat his uncle to death with a fireplace poker for refusing to give him some money. Bogart makes comments about how, 'he's a criminal, they're murderers,' or, 'I'm the murderer, they're maniacs,' while the other two grin or loom intimidatingly. But Bogart also expresses surprise the guard they attacked in their escape survived, which Ustinov attributes to poor prison cuisine sapping their strength.
It's a little odd the movie keeps reminding us of that, but I guess it's to make the ending fit, and also to play into the idea even violent people, who might be considered among the worst, aren't beyond doing some good.