WELLFLEET — The Tokyo String Quartet's opening concert of the Cape Cod Chamber Music Festival Monday evening at the First Congregational Church in Wellfleet offered a study in extremes.

Extremes of emotion: Its appearance was a final, bittersweet chance to enjoy the estimable foursome on the Cape, as the group will retire next June after a worldwide tour.

Extremes of musical expression as well: with a robust program that ranged freely, from the crystalline miniatures of Anton Webern, to the measured inventiveness of Haydn, and culminating in the florid Romantic vision of Schubert.

Tokyo formed in 1969, a time when it was unusual for Asian musicians to explore Western music at all, much less create a quartet that would boldly examine all the major repertory through performance and recordings. Many seasons, and many great performances, have passed. The quartet has changed personnel over the years, and now the two longest tenured members, second violinist Kikuei Ikeda and violist Kazuhide Isomura, have decided to step off the stage. Cellist Clive Greensmith and first violinist Martin Beaver initially considered reforming the group, but in the end decided it was time to retire the ensemble.

Tokyo has accomplished much. Any visit to an American conservatory, heavily populated with aspiring Asian musicians, will offer proof that it's no longer a unique proposition to cross cultures and acquire musical expertise. Tokyo helped make that a reality.

The group recorded all the standard quartet repertory, and then went back and did it again. Multiple cycles of the Beethoven and Bartok quartets, among others, brought a new understanding to those challenging catalogs. A decades-long artistic residency at Yale inspired a new generation of players.

And, not to be forgotten, they have won fans on the Cape. Because of a long professional relationship with co-artistic director Jon Nakamatsu, Tokyo has opened Cape Cod Chamber Music Festival for the past four seasons, and Monday's sold-out hall and general enthusiasm showed that it will be missed.

The group began with Haydn, one of the "Sun" quartets, the D major from opus 20. Haydn molded the form of the genre, but around this mid-point in his creative life, as someone said, all that was left was more genius. The basic structure was in place — sonatalike opening section, a slow movement, a scherzo, and then a rousing conclusion — leaving room for creative investigation.

The D major is remarkable in many ways; of greatest interest is the second movement, a slow theme with four variations. Beaver sketched out the melody: a lovely line with a staccato "hiccup" in the middle. Ikeda took lead on the first variation, which simply took the original melody and smoothed out the "bump." Greensmith boldly introduced the second variant, raising the low voice of the cello up above his counterparts. The third variation, more traditional, had Beaver playing brisk scales above the ensemble. The final section was a slower, introspective version of the first theme, now almost a distant, youthful memory.

The movement seemed like a microcosm of what a quartet can be, and what Tokyo has become: four great musicians, individually brilliant, forming a whole greater than its parts.

Their performance of Webern's Five Movements from opus 5 stood out. Only a few minutes long — with one movement, the third, less than 30 seconds — the work shows Webern at his essence. He despised excess, and believed that a single beautiful sound could serve just as well as a long melodic line.

In just the first few measures, the group played loudly, muted, pizzicato, Bartok pizzicato (snapping the strings), con legno (with the wood of the bow) and sul ponticello (on the bridge). But it was no novelty showpiece of string technique — every sound counted. Attention was rapt, on stage and off. The music is so dense, with no release for the players, that at one point Greensmith had to reach over and turn pages for Isomura — he was too busy. Beaver was in the spotlight more than usual with this repertory, consistent with his flawless intonation.

After intermission, something completely different. For Webern, one note could say everything. For Schubert, one note merely unlocked the door to melodic adventures. His Quartet No. 15, in G major, has many beautiful moments, but is mainly characterized by intense exploration. Ideas are presented and then overturned, dissected, restated and recapitulated. Here was another side of Tokyo: the experienced ambassadors of the great Romantic tradition. Like much of what they have performed in the past four decades, this quartet was challenging on many levels. They made sure the rewards were worth the effort.