If “Ender’s Game” is following a tried-and-true young adult formula this weekend, “Last Vegas” (Rated PG-13; 105 Min.) is doing the opposite: it is the second movie in two weeks, after “Bad Grandpa,” venturing into the brave unknown of the “Seniors Behaving Badly” genre.

Sure, “Betty White’s Off Their Rockers” was a mild success for NBC, and gags about old folks have been rampant in movies for years, but Hollywood studios usually reserved their older-skewing flicks for dramas. “Last Vegas” takes the wildly successful “Hangover” franchise and gives it an AARP spin by sending Billy (Michael Douglas), Paddy (Robert De Niro), Archie (Morgan Freeman) and Sam (Kevin Kline) out to Vegas to celebrate Billy’s bachelor party.

Film 101 teaches aspiring filmmakers every movie involves one of two plots: a stranger comes to town or a hero goes on a journey. “Last Vegas” banks on the former, a fish-out-of-water scenario where old guys end up in the middle of a young man’s world. It is a one-joke comedy that only succeeds because its cast seems to be having so much fun.

Because whether Archie can dance simply is not a good enough conflict, writer Dan Fogelman injects a rift between Billy and Paddy: The two were in love with the same woman while growing up and Paddy ended up winning her over. Of course, there is still bad blood and over the course of the weekend, the two must overcome their differences and learn a lesson along the way.

It is pretty clear Fogelman’s script is quite pedestrian. “Last Vegas” breaks zero new ground beyond throwing some Academy Award winners into a “Hangover” scenario. When the movie meanders on to this entirely predictable side plot “Last Vegas” starts to burn the good will it has built up during its funnier bits. Mary Steenburgen is thrown into the mix as Diana, a lounge singer the

two become interested in. Guess where that side plot is going? It is a shame Steenburgen is not allowed to be much more than a pawn in Billy and Paddy’s feud.

Luckily, there is more good than bad here. Although De Niro and Douglas take the lead, all the time they are swamped in backstory is time for Freeman and Kline to shine. As Sam, Kline steals the show, pure and simple. His wife is looking to spice up their marriage and has given him permission to cheat while in Vegas. Kline uses his charm and wit to really sell his character. It doesn’t hurt that Fogelman throws him the best jokes.

Freeman is always a welcome sight. Here he gets drunk and shows off his dance moves. I repeat: Morgan Freeman gets drunk and shows off his dance moves. This alone is worth the price of admission.

“Last Vegas” works because the audience does not have to be convinced these four established actors are having a good time. Their performances seem almost effortless, even when the dialogue starts to drag them down. They are not here to sell a twisty plot or overwrought drama — they are here to have a good time. The humor, while somewhat corny and always safe, is a welcome respite from comedies that exist only to gross out the audience. “Last Vegas” is driven by star power that never seems to peter out.

Even though the movie does occasionally stray from its real objective, it is fun and engaging more often than not. Audiences will leave the theater with smiles on their faces, and that is certainly not a bad way to kick off the holiday movie season.