The latest M.G.M. musical extravaganza, "Royal Wedding," has the world's longest song title, a pleasant but undistinguished musical score, and Fred Astaire. Astaire alone makes the show worth seeing. At the age of 50 and after several "retirements," he has still enough dancing magic to make an ordinary musical sparkle, but this should come as no surprise. What is news is that Jane Powell, the current in a long list of Astaire dancing partners, sings and dances with enough youthful enthusiasm to rival the old master.

The plot these two must work with is at best ordinary. It concerns a brother and sister team which goes to London for a show amidst the excitement of Princess Elizabeth's wedding. While there, Miss Powell inexplicably falls in love with Peter Lawford who plays a British nobleman. Lawford only proves anew that he just cannot act, even in his native surroundings. Meanwhile, the other half of the team, Fred Astaire, interrupts his dancing long enough to go romantic with Sarah Churchill, who is making her much-heralded debut in American movies. Keenan 'Wynn in the double role of an American press agent and his twin counterpart in Britain tries hard to be funny, but only complicates things.

As the plot is too innocuous to get in the way, spectators are free to concentrate their attention on the song and dance routines. The movie's highlight comes when Astaire and Miss Powell clown their way through a mouthful called "How Could You Believe Me When I Tell You That I Love You When You Know I've Been a Liar All My Life." Also outstanding is an Astaire solo where, aided by a revolving room, he dances on the walls and ceilings in seeming defiance of all laws of gravity.

Astaire's dancing, Jane Powell's charm, and the Technicolor pageantry of London during Elizabeth's wedding more than compensate for a weak plot. As musicals go, "Royal Wedding" is good light-hearted entertainment.