The opera-ballet Wolf-in-Skins — with its ancient Celtic narrative, semi-naked men in wolf's clothing, and antique-style music — is so strange and curious that its workshop at Temple University some five years ago could well have evaporated into the ether like the folk-based stories on which it's based.

Yet it's back Wednesday and Thursday at the Kimmel Center's SEI Innovation Studio, still in a workshop setting — it's being called an "informance" — and with its original team members, some of whom have become much more famous since the last incarnation.

Choreographer/librettist Christopher Williams has been staging baroque opera in Europe and working with A-list directors. Composer Gregory Spears had a hit opera with Fellow Travelers. Countertenor Anthony Roth Costanzo not only sings at major opera companies but now has a significant recording contract.

The opera's story is a prehistory myth about a cursed man who has been changed into a wolf but is brought partway back to humanhood. Composer Spears describes it as "very elaborate, unique, and complex," though for choreographer Williams that distant world seems to be in his blood. Choreographers are known for all manner of muscle memory, and Williams' visual inspiration for this work comes partly from medieval tapestries.

"I have such a clear vision for what is supposed to happen on stage and what it should look like that it's been like a pot simmering," said Williams. "I know exactly what it needs to be. But it's hard to find the right venue, the right producer, the right moment."

Philadelphia Dance Project and American Opera Projects backed both the 2013 workshop of Act I at Temple University and the new Kimmel Center presentation of assorted excerpts. The video of the earlier performance — available to the public on Vimeo — perhaps seems more remarkable when viewed today. To say that the dancers perform on "all fours" is true but hardly captures the kind visual vocabulary that was convincingly developed for the project.

The Celtic mythology could easily fall into quaintness or cuteness but instead comes off a bit ominous and dangerous. The Spears score looks back with all manner of techniques, from medievalesque drone effects to rhapsodic solo-instrument effects worthy of Biber's 17th-century Rosary Sonatas.

Malevolent elements are held at bay by balances of elemental power. Humans sing, animals dance, arias convey thoughts, and choreography conveys action. Transformations that might be called magic or supernatural are taken as a not-unusual part of the landscape in a place where humans and animals procreate.

So arresting was the imagery that the Temple University audiences in the first presentation seemed not to mind that much went unexplained.

In this new incarnation, a beyond-life-size puppet of a bull is seen for the first time — significant both to the piece and to its future development. The full-scale opera-ballet has three kinds of characters: mythological humanoids mainly represented by wolf dancers and shadow figures, real humans from ancient cultures, and beasts such as the bull puppet that will carry key plot points. The puppet itself — known as "the bewitched bull of the lake" — is so big it's operated by two people.

"He's the character that gets the action moving," said Williams. "He's guarded by the maidens — like the Rhine gold in [Wagner's] Ring Cycle — and manages to escape, which causes great havoc. I've never worked with a puppet of this size before."

The production is the dance counterpart to an indie movie — without the backing of a major dance, theater, or opera company — and Williams has been courting the support of the Jim Henson Foundation for the puppetry element. The new bull should enhance his eligibility. But what might be truly needed to finish the piece is a government-subsidized European theater, the sort that has allowed theater artists such as Robert Wilson to create large, full-evening works for decades.

Truth is, there's really no template for the piece — and its creation often requires the opposite of the usual dynamics. Typically, librettists work within the world the composer is trying to create. Not here. "Christopher would hand me the libretto," says Spears, "and I would have to figure out how to support it. When an aria is four stanzas of poetry … and it isn't about everyday conversations between ordinary people … it requires a different kind of music."

Much of his inspiration is from the 17th-century operas that attempted to recapture the sung dramas of ancient times. But later 18th-century opera comes into play, as well. "I love what an aria can do — being able to stop time within a sentiment, not about discovering something in particular but being in the sentiment."

Between periods of working on Wolf-in-Skins, the team — all in their mid-30s to early 40s — diverges into hugely different works that draw on their rich backgrounds. Baroque opera, for one, requires Williams to work in a far more formal choreographic framework. Spears dramatized emotions much closer to the surface in his opera Fellow Travelers, about clandestine gay lovers in 1950s Washington. Countertenor Costanzo is a former child star from Broadway's Falsettos and has enjoyed recent breakthroughs playing an Egyptian pharaoh in Philip Glass' Akhnaten — and would normally sing in theaters larger than the Kimmel Center's SEI Innovation Studio.

Yet they reconnect, thanks partly to their own personal histories.

"Christopher and Anthony were two of the first musicians I started working with when I came to New York," says Spears. "We're good friends. You get together with your friends and make art."

"It doesn't matter that there's been this incredible drift of time," said Williams. "It's only a richer connection now."

Wolf-in-Skins

Workshop performances at 7:30 p.m. Wednesday and Thursday in the SEI Innovation Studio at the Kimmel Center, 300 S. Broad St.

Tickets: $10

Information: 215-893-1999 or www.kimmelcenter.org.