The present Code of Canon Law reads: "On Sundays and other holy days of obligation the faithful are bound to participate in the Mass." It doesn't say "part" or "parts" of the Mass. The expectation is that the person will attend a complete Mass. A Catholic Catechism quotes the canon and states, "Those who deliberately fail in this obligation commit a grave sin."
Before Vatican II, moral theologians and canonists would talk about the three principal parts of Mass as the Offertory, Consecration and Communion. If you missed any one of those parts, they wrote, you would not have fulfilled the obligation of hearing Mass.
Today, canonists and liturgists do not use that terminology. They speak of the gathering, the Liturgy of the Word, the Liturgy of the Eucharist and the commissioning as the main divisions of Mass.
And moralists are more likely to speak of substantial observance of the law and what that might mean. They would assert that the law imposes a serious obligation. But some would question whether a person seriously or gravely violates the law if on one occasion he or she does not attend Sunday Mass. And all moralists would acknowledge that to miss a few minutes would not be a serious matter.
If you look at your missalette or recall your experience on Sundays, the penitential rite is part of the Mass. It takes place after the entrance song, right after the priest has entered the sanctuary and greeted the people. It can take different forms. One commonly used is the confession of fault (Confiteor) and Lord, have mercy (Kyrie). So if you come after these prayers, you are late for Mass.