A Funny Thing Happened on the Way to the Forum, kicking off the 2017-2018 season at the Walnut Street Theatre (through Oct. 22, a long run for such a big show), is what it is. As in 1963. A farce based on the comedy of the Roman playwright Plautus, it also banks on burlesque, vaudeville, cabaret, slapstick, and wacky humor — as in Marx Brothers, Three Stooges, scantily clad shimmy-shakers, mugging, pratfalls, stereotypes in all directions. Pretty girls (not women)? Airheads. Old guys? Randy yet impotent. Soldiers? Brainless drones and inflated gasbags.

And that is why, at the end, my face muscles, diaphragm, and throat all hurt: I'd been groaning and laughing for two-hours-plus. So had a packed, delighted house. Old-fashioned, inappropriate, vulgar as can be, this Forum is about as good as the thing can be done: It's headlong, merry, madcap, paralytically funny, and full of life. Directed by and starring farcemaster Frank Ferrante as wily slave Pseudolus ("Liar"), he and his enormous cast are totally committed to silly, pointless, nutso foolery.

Three great examples are the Proteans, a mini-ensemble who play trios of eunuchs, soldiers, Romans, chorus members, whatever you got. Hail, comic geniuses Dave Jadico, Jennie Eisenhower, and Ben Dibble. What faces! They play it to the hilt, then they find another hilt and play it to that, hilt after hilt.

Pseudolus is a slave who wants to be free. Ferrante's rendition of "Free" is actually touching for a moment, then, in Stephen Sondheim's brilliant words: "Not so fast. … I didn't think. … The way I am, I have a roof, three meals a day, and I don't have to pay a thing, I'm just a slave, and everything's free. If I were free, then nothing would be free. … "

When Pseudolus learns that his master's son, Hero (the winning Brandon O'Rourke), is in love with the girl next door (next door being a brothel), he strikes a bargain: his freedom for the girl. Thus begins a chain of machinations, impostures, and stratagems, including mare's sweat, pirates, cross-dressing, and mistaken identity. Some Philly greats are great again here: Fran Prisco as the pimp Lycus (pronounced "Like Us"), Mary Martello as old battle-ax Domina, and especially Scott Greer as Hysterium, the loyal, stuck-up head slave ripe for humiliation. His pas de deux with Martello in "That Dirty Old Man" defeats description.

Alanna J. Smith plays the girl, Philia, pert, stratospheric soprano, and hilarious in her ignorance. Raised to be a courtesan, she's still a virgin, awaiting Miles Gloriosus ("Braggart Soldier"), who has purchased her. So she knows all about sex (which she has never experienced) and nothing else whatever. The orchestra is wonderfully tight, keeping up the sparkle and fun in some tricky music. Ferrante's ad-libs brought down the house. When he and Greer messed up Latin names, Ferrante reached into the audience for a playbill to check. To Greer: "We may replace you with the inflatable rat outside" — a nod to the union picket line in front of the Walnut.

The last 20 minutes are especially inspired, with citizens of all degrees chasing and being chased throughout the glittering streets of Rome. It has a timeless resonance, as if this were what Plautus, Sondheim, and Ferrante and cast were after all along.

A Funny Thing Happened on the Way to the Forum. Through Oct. 22 at the Walnut Street Theatre, 825 Walnut St. Tickets: $20-$97. Information: 215-574-3550, walnutstreettheatre.org